Zoom lens system, imaging device and camera

ABSTRACT

An object of the present invention is to provide: a zoom lens system that has a high resolution, high capability of compensating curvature of field, a high zoom ratio of, for example, 3 or greater, a reduced weight, and a reduced overall optical length at the time of non-use; and an imaging device and a camera that employ this zoom lens system so as to have a reduced thickness and excellent portability as well as high performance. The zoom lens system forms an optical image of an object with variable magnification and, in order from the object side to the image side, comprises: a first lens unit having negative optical power; a second lens unit that has positive optical power and that contains a bi-convex lens element composed of a resin material and serving as the most image side lens element; and a third lens unit having positive optical power. Then, the lens units move respectively along the optical axis in such a manner that intervals between the individual lens units should vary so that the variable magnification is achieved. Further, the condition |(R 2mi1 +R 2mi2 )/(R 2mi1 −R 2mi2 )|&lt;1.0 (R 2mi1  is a radius of curvature on the object side of the most image side lens element of the second lens unit, while R 2mi2  is a radius of curvature on the image side of the most image side lens element of the second lens unit) is satisfied. The imaging device and the camera employ this zoom lens system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system, an imaging deviceand a camera. In particular, the present invention relates to a zoomlens system that has a high resolution, high capability of compensatingcurvature of field, a high zoom ratio of, for example, 3 or greater, areduced weight, and a reduced overall optical length at the time ofnon-use; and an imaging device and a camera that employ the zoom lenssystem so as to have a reduced thickness and excellent portability aswell as high performance.

2. Description of the Background Art

In the prior art, a large number of optical instruments have beendeveloped that form an image of a photographic object onto an imagesensor through a lens and then acquire the object image as an image.Recently, products such as digital still cameras and digital videocameras are spreading. Then, with the increase in the number of users,desire on these products is also growing. Among various types of theseproducts, optical instruments having a zoom ratio of approximately 3 arerelatively small and still have an optical zoom function. Thus, thesetypes are spreading remarkably widely as digital cameras of compact typeor stylish type.

In the digital cameras of compact type, for the purpose of the propertyof easy carrying, further size reduction of the instruments is desired.In order to achieve the further size reduction of the digital cameras,the lens arrangement need be adopted such that the overall opticallength (the distance measured from the top of the most object side lenssurface of the entire lens system to the image surface) at the time ofnon-use should be reduced while lens elements that extend out relativeto the main body by means of a multi-stage lens barrel at the time ofuse could be accommodated into the main body. Further, in the digitalcameras to spread widely, cost reduction is also desired.

Meanwhile, as zoom lens systems suitable for digital still cameras ofcompact type, a large number of zoom lens systems of three-unitconstruction have been proposed that, for example, in order, form theobject side to the image side, comprise a first lens unit havingnegative optical power, a second lens unit having positive opticalpower, and a third lens unit having positive optical power.

In such a zoom lens system of three-unit construction, in zooming(magnification change) from a wide-angle limit to a telephoto limit, theair space between the first lens unit and the second lens unit decreasesmonotonically, while the air space between the second lens unit and thethird lens unit varies, and while the third lens unit is fixed or moved.

Focus adjustment in the zoom lens system of three-unit construction isperformed by moving the first lens unit or the third lens unit in theoptical axis direction. In particular, from the perspective of sizereduction of the entire optical instrument, in many cases, the focusadjustment is performed using the third lens unit which is less heavy,so that focusing onto the photographic object is achieved ranging fromthe infinity to a short distance. In contrast, when the focus adjustmentis performed using the first lens unit, the first lens unit is largerthan the third lens unit and hence requires a large size motor. Thiscauses a tendency of size increase in the entire optical instrument.

The third lens unit having positive optical power has the effects ofcompensating curvature of field and bringing into a telecentric statethe incident light onto the imaging surface. Further, in many cases, thethird lens unit is composed of one or two lens elements having a smallouter diameter, and hence can be driven at a high speed using a smallsize motor. Thus, when the third lens unit is adopted as a lens unit forfocus adjustment, an optical instrument is realized that has a reducedsize and permits rapid focusing.

The first lens unit and the second lens unit move in parallel to theoptical axis along a cam groove formed in a cylindrical cam. In the camgroove, a groove for zooming and a groove for the time of non-use areconnected to each other. The groove for the time of non-use reduces theinterval between the lens units and moves all the three lens units tothe image sensor side. This configuration reduces the overall opticallength at the time of non-use. In this case, if the thickness of eachlens unit could be reduced, the overall optical length at the time ofnon-use would be reduced further.

As such, in the prior art, design has been performed such that the zoomlens system should have the above-mentioned configuration where the sizeis reduced in the part relevant to focus adjustment and in the entirelens system at the time of non-use, so that the overall optical lengthof the digital still camera has been reduced and so has been the cost.

For example, Japanese Laid-Open Patent Publication No. 2006-10895discloses a three-unit zoom lens, in order from the object side to theimage side, comprising: a first lens unit having negative optical powerwhich is, composed of a negative power lens and a positive power lens; asecond lens unit having positive optical power which is composed of acemented lens consisting of a convex lens and a concave lens and asingle lens; and a third lens unit having positive optical power. Inthis three-unit zoom lens, an aspheric surface is employed in the firstlens unit and in the second lens unit, while a synthetic resin lens isemployed as the single lens of the second lens unit and in the thirdlens unit. By virtue of this, in a state that aberration is compensatedsatisfactorily, the individual lens thicknesses are reduced so that sizereduction and cost reduction are achieved in the optical system.

Further, for example, in a zoom lens disclosed in Japanese Laid-OpenPatent Publication No. 2004-144947, two positive lenses are provided onthe image side relative to the second unit so that the optical power perlens is reduced. Then, these lenses having relatively low optical powerare composed of plastic lenses. In general, in plastic lenses, the imagesurface position and the like easily fluctuate owing to environmentalvariation. However, in the zoom lens disclosed in Japanese Laid-OpenPatent Publication No. 2004-144947 where such plastic lenses havingrelatively low optical power are employed, influence of environmentalvariation is rather small, and hence view angle enhancement in theshooting view angle and weight reduction in the lens system are achievedsimultaneously.

Further, for example, in a zoom lens disclosed in Japanese Laid-OpenPatent Publication No. 2002-372667, an aspheric surface is employed inthe most image side lens of the second lens unit, so that astigmatism iscompensated. Further, within the second lens unit, at least a lensarrangement of positive, positive and negative in order from the objectside is included. Further, an aspheric plastic lens is arranged on themost image side. By virtue of this, spherical aberration and comaaberration are compensated, while the Petzval sum is reduced so thatcurvature of field is reduced. As such, in the zoom lens disclosed inJapanese Laid-Open Patent Publication No. 2002-372667, aberrationgenerated in the second lens unit is reduced so that aberrationfluctuation during zooming is reduced. Further, the optical power in onelens unit is divided so that the optical power per lens is reduced. Thisreduces spherical aberration and coma aberration generated in thepositive lenses. Moreover, since a plastic lens is employed, injectionmolding is adopted and permits easy aspheric surface formation,manufacturing error reduction, and weight reduction in the entire lens.

Further, for example, in a variable magnification optical systemdisclosed in Japanese Laid-Open Patent Publication No. 2006-11096, atleast one aspheric surface is employed in the first lens unit so as tocompensate astigmatism and distortion generated when the negativeoptical power of the first lens unit is increased. This permits sizereduction. Further, in the first lens unit, a lower limit is placed onthe absolute value of the refractive index difference of the lenses, sothat the Petzval sum is reduced. Then, astigmatism and curvature offield are compensated.

In the configuration of the variable magnification optical systemdisclosed in Japanese Laid-Open Patent Publication No. 2006-11096, anegative lead is adopted in the first lens unit on the most object sidesuch that that the light beam incident at a large angle from the objectside to the lens surface should be refracted greatly, so that sizereduction is achieved. Further, a lower limit is placed on the maximumvalue of refractive index in the lenses within the first lens unit. Thisreduces the radii of curvature of the lenses, and hence suppresses anincrease in the generated aberration. Moreover, a limit is placed on theoptical power of the second lens unit, so that an increase is suppressedin decentration error sensitivity and manufacturing difficulty generatedin the second lens unit in a state that the variable magnification ratioand compactness are maintained satisfactorily.

Further, for example, in a zoom lens disclosed in Japanese Laid-OpenPatent Publication No. 2006-194974, a two-lens construction is adoptedin the first lens unit so that the size in the retracted state isreduced and hence overall thickness reduction is achieved. Further,various kinds of aberration generated in the first lens unit inassociation with the thickness reduction are suppressed by increasingthe refractive indices of the individual lenses and increasing thecurvatures of the lens surfaces. In particular, axial chromaticaberration and magnification chromatic aberration are suppressed byplacing upper and lower limits on the refractive index differencebetween the second lens and the first lens in the first lens unit and onthe Abbe number of the second lens.

Nevertheless, in the configuration of the three-unit zoom lens disclosedin Japanese Laid-Open Patent Publication No. 2006-10895, the image sidepositive power lens of the first lens unit has a low refractive indexand still is a spherical lens. This causes a problem of insufficiency inthe compensation of curvature of field.

Further, in the three-unit zoom lens disclosed in Japanese Laid-OpenPatent Publication No. 2006-10895, when the focal length of the firstlens unit is set up shorter for the purpose of size reduction, thediameter of the lens can be constructed relatively small. Nevertheless,when the first lens unit is constructed from two lenses, the opticalpower of the object side lens need be increased, and hence the thicknessof the image side lens also need be increased for the purpose ofcompensation of chromatic aberration. This causes a problem of increasein the overall optical length at the time of non-use.

In the zoom lens of three-unit construction disclosed in JapaneseLaid-Open Patent Publication No. 2004-144947, the optical power of theplastic lenses has a high ratio within the optical power of the entiresecond unit. Thus, when the optical power of the plastic lens is to bemaintained low, the optical power of the entire second unit also becomeslow. Thus, for the purpose of view angle enhancement, the amount ofmovement of the second unit during variable magnification need beincreased. This causes difficulty in thickness reduction of the entirezoom lens.

In the zoom lens of three-unit construction disclosed in JapaneseLaid-Open Patent Publication No. 2002-372667, since a plastic lens isemployed, weight reduction and manufacturing error reduction areachieved to some extent. Nevertheless, these weight reduction andmanufacturing error reduction are not satisfactorily achievedsimultaneously to improvement in the optical performance and thicknessreduction of the entire zoom lens.

In the variable magnification optical system of three-unit constructiondisclosed in Japanese Laid-Open Patent Publication No. 2006-11096,importance is imparted to the compactness. Thus, a problem ofinsufficient compensation of curvature of field is present. Further, theoptical power of the second lens unit is low, and hence a zoom ratio of3 or greater is difficult to be achieved.

In the zoom lens of three-unit construction disclosed in JapaneseLaid-Open Patent Publication No. 2006-194974, compensation ofmagnification chromatic aberration is performed mainly by adjusting therefractive indices and the Abbe numbers of the lenses in the first lensunit as well as by adopting a cemented lens for the second lens unit.Nevertheless, compensation of various kinds of aberration isinsufficient for the purpose of achieving a zoom ratio of 3 or greater.Further, view angle enhancement is also difficult to be achieved.

SUMMARY OF THE INVENTION

The present invention has been made in order to resolve the problemsdescribed in the background art. An object of the present invention isto provide: a zoom lens system that has a high resolution, highcapability of compensating curvature of field, a high zoom ratio of forexample 3 or greater, a reduced weight, and a reduced overall opticallength at the time of non-use; and an imaging device and a camera thatemploy this zoom lens system so as to have a reduced thickness andexcellent portability as well as high performance.

(I) The above-mentioned object is achieved by a zoom lens system, animaging device and a camera described below. That is, the presentinvention relates to:

a zoom lens system that forms an optical image of an object withvariable magnification

and that, in order from the object side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit that has positive optical power and that contains abi-convex lens element composed of a resin material and serving as themost image side lens element; and

a third lens unit having positive optical power, wherein

the lens units move respectively along an optical axis in such a mannerthat intervals between the individual lens units should vary so that thevariable magnification is achieved, and the following condition (I-1) issatisfied:

|(R _(2mi1) +R _(2mi2))/(R _(2mi1) −R _(2mi2))|<1.0  (I-1)

where,

R_(2mi1) is a radius of curvature on the object side of the most imageside lens element of the second lens unit, and

R_(2mi2) is a radius of curvature on the image side of the most imageside lens element of the second lens unit;

an imaging device capable of converting an optical image of aphotographic object into an electric image signal and then outputtingthe signal, comprising:

a zoom lens system that forms the optical image of the photographicobject with variable magnification; and

an image sensor that converts the optical image formed by the zoom lenssystem into the electric image signal, wherein

in the zoom lens system,

the system, in order from the object side which is the photographicobject side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit that has positive optical power and that contains abi-convex lens element composed of a resin material and serving as themost image side lens element; and

a third lens unit having positive optical power, wherein

the lens units move respectively along an optical axis in such a mannerthat intervals between the individual lens units should vary so that thevariable magnification is achieved, and

the following condition (I-1) is satisfied:

|(R _(2mi1) +R _(2mi2))/(R _(2mi1) −R _(2mi2))|<1.0  (I-1)

where,

R_(2mi1) is a radius of curvature on the object side of the most imageside lens element of the second lens unit, and

R_(2mi2) is a radius of curvature on the image side of the most imageside lens element of the second lens unit; and

a camera capable of shooting a photographic object and then outputtingits image as an electric image signal, comprising

an imaging device including a zoom lens system that forms the opticalimage of the photographic object with variable magnification and animage sensor that converts the optical image of the photographic objectformed by the zoom lens system into the electric image signal, wherein

in the zoom lens system,

the system, in order from the object side which is the photographicobject side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit that has positive optical power and that contains abi-convex lens element composed of a resin material and serving as themost image side lens element; and

a third lens unit having positive optical power, wherein

the lens units move respectively along an optical axis in such a mannerthat intervals between the individual lens units should vary so that thevariable magnification is achieved, and

the following condition (I-1) is satisfied:

|(R _(2mi1) +R _(2mi2))/(R _(2mi1) −R _(2mi2))|<1.0  (I-1)

where,

R_(2mil) is a radius of curvature on the object side of the most imageside lens element of the second lens unit, and

R_(2mi2) is a radius of curvature on the image side of the most imageside lens element of the second lens unit.

(II) The above-mentioned object is achieved by a zoom lens system, animaging device and a camera described below. That is, the presentinvention relates to:

a zoom lens system that forms an optical image of an object withvariable magnification

and that, in order from the object side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit having positive optical power; and

a third lens unit having positive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit shoulddecrease while an interval between the second lens unit and the thirdlens unit should vary so that the variable magnification is achieved,

the first lens unit is composed of one object side lens element and oneimage side lens element,

the image side lens element of the first lens unit has positive opticalpower,

the most image side lens element constituting the second lens unit is alens element composed of a resin and having at least one asphericsurface, and

the following condition (II-1) is satisfied:

0.7<f ₂ /f _(2r)<1.5  (II-1)

where,

f₂ is a composite focal length of the second lens unit, and

f_(2r) is a focal length of the most image side lens element of thesecond lens unit;

an imaging device capable of converting an optical image of aphotographic object into an electric image signal and then outputtingthe signal, comprising:

a zoom lens system that forms the optical image of the photographicobject with variable magnification; and

an image sensor that converts the optical image formed by the zoom lenssystem into the electric image signal, wherein

in the zoom lens system,

the system, in order from the object side which is the photographicobject side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit having positive optical power; and

a third lens unit having positive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first-lens unit and the second lens unit shoulddecrease while an interval between the second lens unit and the thirdlens unit should vary so that the variable magnification is achieved,

the first lens unit is composed of one object side lens element and oneimage side lens element,

the image side lens element of the first lens unit has positive opticalpower,

the most image side lens element constituting the second lens unit is alens element composed of a resin and having at least one asphericsurface, and

the following condition (II-1) is satisfied:

0.7<f ₂ /f _(2r)<1.5  (II-1)

where,

f₂ is a composite focal length of the second lens unit, and

f_(2r) is a focal length of the most image side lens element of thesecond lens unit; and

a camera capable of shooting a photographic object and then outputtingits image as an electric image signal, comprising

an imaging device including a zoom lens system that forms the opticalimage of the photographic object with variable magnification and animage sensor that converts the optical image of the photographic objectformed by the zoom lens system into the electric image signal, wherein

in the zoom lens system,

the system, in order from the object side which is the photographicobject side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit having positive optical power; and

a third lens unit having positive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit shoulddecrease while an interval between the second lens unit and the thirdlens unit should vary so that the variable magnification is achieved,

the first lens unit is composed of one object side lens element and oneimage side lens element,

the image side lens element of the first lens unit has positive opticalpower,

the most image side lens element constituting the second lens unit is alens element composed of a resin and having at least one asphericsurface, and

the following condition (II-1) is satisfied:

0.7<f ₂ /f _(2r)<1.5  (II-1)

where,

f₂ is a composite focal length of the second lens unit, and

f_(2r) is a focal length of the most image side lens element of thesecond lens unit.

(III) The above-mentioned object is achieved by a zoom lens system, animaging device and a camera described below. That is, the presentinvention relates to:

a zoom lens system that forms an optical image of an object withvariable magnification of a factor of 3 or greater

and that, in order from the object side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit having positive optical power; and

a third lens unit having positive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit shoulddecrease while an interval between the second lens unit and the thirdlens unit should vary so that the variable magnification is achieved,

the first lens unit is composed of one object side lens element and oneimage side lens element,

in the first lens unit, the object side lens element has negativeoptical power while the image side lens element has positive opticalpower, and

the following condition (III-1) is satisfied:

1.5<f ₂ /f _(W)<2.8  (III-1)

(here, ω_(W)>36)

where,

f₂ is a composite focal length of the second lens unit,

f_(W) is a focal length of the entire system at a wide-angle limit, and

ω_(W) is a half view angle at a wide-angle limit;

an imaging device capable of converting an optical image of aphotographic object into an electric image signal and then outputtingthe signal, comprising:

a zoom lens system that forms the optical image of the photographicobject with variable magnification of a factor of 3 or greater; and

an image sensor that converts the optical image formed by the zoom lenssystem into the electric image signal, wherein

in the zoom lens system,

the system, in order from the object side which is the photographicobject side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit having positive optical power; and

a third lens unit having positive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit shoulddecrease while, an interval between the second lens unit and the thirdlens unit should vary so that the variable magnification is achieved,

the first lens unit is composed of one object side lens element and oneimage side lens element,

in the first lens unit, the object side lens element has negativeoptical power while the image side lens element has positive opticalpower, and

the following condition (III-1) is satisfied:

1.5<f ₂ /f _(W)<2.8  (III-1)

(here, ω_(W)>36)

where,

f₂ is a composite focal length of the second lens unit,

f_(W) is a focal length of the entire system at a wide-angle limit, and

ω_(W) is a half view angle at a wide-angle limit; and

a camera capable of shooting a photographic object and then outputtingits image as an electric image signal, comprising

an imaging device including a zoom lens system that forms the opticalimage of the photographic object with variable magnification of a factorof 3 or greater and an image sensor that converts the optical image ofthe photographic object formed by the zoom lens system into the electricimage signal, wherein

in the zoom lens system,

the system, in order from the object side which is the photographicobject side to the image side, comprises:

a first lens unit having negative optical power;

a second lens unit having positive optical power; and

a third lens unit having positive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit shoulddecrease while an interval between the second lens unit and the thirdlens unit should vary so that the variable magnification is achieved,

the first lens unit is composed of one object side lens element and oneimage side lens element,

in the first lens unit, the object side lens element has negativeoptical power while the image side lens element has positive opticalpower, and

the following condition (III-1) is satisfied:

1.5<f ₂ /f _(W)<2.8  (III-1)

(here, ω_(W)>36)

where,

f₂ is a composite focal length of the second lens unit,

f_(W) is a focal length of the entire system at a wide-angle limit, and

ω_(W) is a half view angle at a wide-angle limit.

According to the present invention, a zoom lens system is provided thathas a high resolution, high capability of compensating curvature offield, a high zoom ratio of, for example, 3 or greater, a reducedweight, and a reduced overall optical length at the time of non-use.Further, the present invention provides: an imaging device that employsthis zoom lens system so as to have a reduced thickness and excellentportability as well as high performance: and a camera such as a digitalstill camera and a digital video camera that has a reduced thickness andexcellent portability as well as high performance.

These and other objects, features, aspects and effects of the presentinvention will become clearer on the basis of the following detaileddescription with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of a zoom lens system according toEmbodiment I-1 (Example I-1);

FIG. 2 is a longitudinal aberration diagram of a zoom lens systemaccording to Example I-1;

FIG. 3 is a configuration diagram of a zoom lens system according toEmbodiment I-2 (Example I-2);

FIG. 4 is a longitudinal aberration diagram of a zoom lens systemaccording to Example I-2;

FIG. 5 is a configuration diagram of a zoom lens system according toEmbodiment I-3 (Example I-3);

FIG. 6 is a longitudinal aberration diagram of a zoom lens systemaccording to Example I-3;

FIG. 7 is a configuration diagram of a zoom lens system according toEmbodiment I-4 (Example I-4);

FIG. 8 is a longitudinal aberration diagram of a zoom lens systemaccording to Example I-4;

FIG. 9 is a configuration diagram of a zoom lens system according toEmbodiment II-1 (Example II-1);

FIG. 10 is a longitudinal aberration diagram of a zoom lens systemaccording to Example II-1;

FIG. 11 is a configuration diagram of a zoom lens system according toEmbodiment II-2 (Example II-2);

FIG. 12 is a longitudinal aberration diagram of a zoom lens systemaccording to Example II-2;

FIG. 13 is a configuration diagram of a zoom lens system according toEmbodiment II-3 (Example II-3);

FIG. 14 is a longitudinal aberration diagram of a zoom lens systemaccording to Example II-3;

FIG. 15 is a configuration diagram of a zoom lens system according toEmbodiment II-4 (Example II-4);

FIG. 16 is a longitudinal aberration diagram of a zoom lens systemaccording to Example II-4;

FIG. 17 is a configuration diagram of a zoom lens system according toEmbodiment III-1 (Example III-1);

FIG. 18 is a longitudinal aberration diagram of a zoom lens systemaccording to Example III-1;

FIG. 19 is a configuration diagram of a zoom lens system according toEmbodiment III-2 (Example III-2);

FIG. 20 is a longitudinal aberration diagram of a zoom lens systemaccording to Example III-2;

FIG. 21 is a configuration diagram of a zoom lens system according toEmbodiment III-3 (Example III-3);

FIG. 22 is a longitudinal aberration diagram of a zoom lens systemaccording to Example III-3;

FIG. 23 is a configuration diagram of a zoom lens system according toEmbodiment III-4 (Example III-4);

FIG. 24 is a longitudinal aberration diagram of a zoom lens systemaccording to Example III-4; and

FIG. 25 is a schematic perspective view of a digital still cameraaccording to Embodiment 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments I-1 to I-4

FIG. 1 is a lens configuration diagram of a zoom lens system accordingto Embodiment I-1. FIG. 3 is a lens configuration diagram of a zoom lenssystem according to Embodiment I-2. FIG. 5 is a lens configurationdiagram of a zoom lens system according to Embodiment I-3. FIG. 7 is alens configuration diagram of a zoom lens system according to EmbodimentI-4. Each configuration diagram shows a zoom lens system in an infinityin-focus condition. In each diagram, part (a) shows a lens configurationat a wide-angle limit (in the minimum focal length condition: focallength f_(W)). Part (b) shows a lens configuration at an approximatelymiddle position (in an intermediate focal length condition: focal lengthf_(M)=√{square root over ( )}(f_(W)*f_(T))). Part (c) shows a lensconfiguration at a telephoto limit (in the maximum focal lengthcondition: focal length f_(T)).

Each zoom lens system according to Embodiments I-1 to I-4, in order fromthe object side to the image side, comprises: a first lens unit G1having negative optical power; a diaphragm A; a second lens unit G2having positive optical power; and a third lens unit G3 having positiveoptical power. In the zoom lens system according to Embodiments I-1 toI-4, in zooming from a wide-angle limit to a telephoto limit, the firstlens unit G1 moves with locus of a convex to the image side, while thesecond lens unit G2 and the diaphragm A monotonically move to the objectside, and while the third lens unit G3 monotonically moves to the imageside. As such, in the zoom lens system according to Embodiments I-1 toI-4, in zooming from a wide-angle limit to a telephoto limit, the lensunits move respectively along the optical axis in such a manner that theintervals between the individual lens units should vary.

Further, in the zoom lens system according to Embodiments I-1 to I-4,magnification change is performed mainly by the first lens unit G1 andthe second lens unit G2, while focusing is performed by the third lensunit G3. Further, the diaphragm A is arranged on the object side of thesecond lens unit G2 and located at the same position as the most objectside surface of the second lens unit G2. This permits reduction in theinterval between the first lens unit G1 and the second lens unit G2, andhence provides an advantage in thickness reduction of the entire zoomlens system and ensuring of magnification.

In FIGS. 1, 3, 5 and 7, the straight line located on the most right-handside indicates the position of the image surface S of an image sensorsuch as a CCD. On the object side relative to this, two plates P areprovided each of which is composed of a parallel plate or a cover glassplate equivalent to an optical low-pass filter or a face plate of animage sensor or the like.

The above-mentioned optical low-pass filter is constructed from amaterial such as quartz having birefringence. For example, in asolid-state image sensor such as a CCD, an object image formed by thezoom lens system is acquired as a two-dimensional sampling image of lownumerical aperture. Thus, a high frequency signal at or higher than ½ ofthe sampling frequency forms a false signal. Accordingly, in order thatsuch high frequency components of the image should be removed inadvance, it is preferable that the optical low-pass filter is arrangedbetween the most image side lens element of the third lens unit G3 andthe image surface S. Further, a solid-state image sensor generally has ahigh sensitivity even for light in the infrared region. Thus, in orderthat natural color reproduction should also be achieved, it is morepreferable that the optical low-pass filter is provided with an infraredcut-off function of cutting off the light in the infrared region bymeans of fabricating the filter with an infrared absorbing material,coating the filter with an infrared absorbing material, or the like.

The above-mentioned plate P is not indispensable, and hence may beprovided appropriately when necessary.

As shown in FIG. 1, in the zoom lens system according to Embodiment I-1,the first lens unit G1, in order from the object side to the image side,comprises two lens elements consisting of: a negative meniscus firstlens element L1 with the convex surface facing the object side; and apositive meniscus second lens element L2 with the convex surface facingthe object side. In the first lens element L1, the image side surface(surface 2) is aspheric. In the second lens element L2, the object sidesurface (surface 3) is aspheric.

Further, in the zoom lens system according to Embodiment I-1, the secondlens unit G2, in order from the object side to the image side, comprisesthree lens elements consisting of a bi-convex third lens element L3(lens element A), a bi-concave fourth lens element L4 (lens element B)and a bi-convex fifth lens element L5 (most image side lens element,lens element C). Among these, the third lens element L3 and the fourthlens element L4 are cemented with each other so as to constitute acemented lens element.

In the zoom lens system according to the present Embodiment I-1, thethird lens element L3 and the fourth lens element L4 are composed of aglass material, while the fifth lens element L5 is composed of a resinmaterial such as an acrylic resin. Further, in the third lens elementL3, the object side surface (surface 6) is aspheric. In the fifth lenselement L5, the object side surface (surface 9) is aspheric.

Further, in the zoom lens system according to Embodiment I-1, the thirdlens unit G3 comprises solely a bi-convex sixth lens element L6. Thesixth lens element L6 is composed of a glass material. Its two surfaces(surfaces 11 and 12) are aspheric.

As shown in FIG. 3, in the zoom lens system according to Embodiment I-2,the first lens unit G1, in order from the object side to the image side,comprises two lens elements consisting of: a negative meniscus firstlens element L1 with the convex surface facing the object side; and apositive meniscus second lens element L2 with the convex surface facingthe object side. In the first lens element L1, the image side surface(surface 2) is aspheric. In the second lens element L2, the object sidesurface (surface 3) is aspheric.

Further, in the zoom lens system according to Embodiment I-2, the secondlens unit G2, in order from the object side to the image side, comprisesthree lens elements consisting of a bi-convex third lens element L3(lens element A), a bi-concave fourth lens element L4 (lens element B)and a bi-convex fifth lens element L5 (most image side lens element,lens element C). Among these, the third lens element L3 and the fourthlens element L4 are cemented with each other so as to constitute acemented lens element.

In the zoom lens system according to the present Embodiment I-2, thethird lens element L3 and the fourth lens element L4 are composed of aglass material, while the fifth lens element L5 is composed of a resinmaterial such as an acrylic resin. Further, in the third lens elementL3, the object side surface (surface 6) is aspheric. In the fifth lenselement L5, the object side surface (surface 9) is aspheric.

Further, in the zoom lens system according to Embodiment I-2, the thirdlens unit G3 comprises solely a bi-convex sixth lens element L6. Thesixth lens element L6 is composed of a glass material. Its two surfaces(surfaces 11 and 12) are aspheric.

As shown in FIG. 5, in the zoom lens system according to Embodiment I-3,the first lens unit G1, in order from the object side to the image side,comprises two lens elements consisting of: a negative meniscus firstlens element L1 with the convex surface facing the object side; and apositive meniscus second lens element L2 with the convex surface facingthe object side. In the first lens element L1, the image side surface(surface 2) is aspheric. In the second lens element L2, the image sidesurface (surface 4) is aspheric.

Further, in the zoom lens system according to Embodiment I-3, the secondlens unit G2, in order from the object side to the image side, comprisesthree lens elements consisting of a bi-convex third lens element L3(lens element A), a bi-concave fourth lens element L4 (lens element B)and a bi-convex fifth lens element L5 (most image side lens element,lens element C). Among these, the third lens element L3 and the fourthlens element L4 are cemented with each other so as to constitute acemented lens element.

In the zoom lens system according to the present Embodiment I-3, thethird lens element L3 and the fourth lens element L4 are composed of aglass material, while the fifth lens element L5 is composed of a resinmaterial such as an acrylic resin. Further, in the third lens elementL3, the object side surface (surface 6) is aspheric. In the fifth lenselement L5, the object side surface (surface 9) is aspheric.

Further, in the zoom lens system according to Embodiment I-3, the thirdlens unit G3 comprises solely a bi-convex sixth lens element L6. Thesixth lens element L6 is composed of a glass material. Its image sidesurface (surface 12) is aspheric.

As shown in FIG. 7, in the zoom lens system according to Embodiment I-4,the first lens unit G1, in order from the object side to the image side,comprises two lens elements consisting of: a negative meniscus firstlens element L1 with the convex surface facing the object side; and apositive meniscus second lens element L2 with the convex surface facingthe object side. In the first lens element L1, the image side surface(surface 2) is aspheric. In the second lens element L2, the object sidesurface (surface 3) is aspheric.

Further, in the zoom lens system according to Embodiment I-4, the secondlens unit G2, in order from the object side to the image side, comprisesthree lens elements consisting of a bi-convex third lens element L3(lens element A), a bi-concave fourth lens element L4 (lens element B)and a bi-convex fifth lens element L5 (most image side lens element,lens element C). Among these, the third lens element L3 and the fourthlens element L4 are cemented with each other so as to constitute acemented lens element.

In the zoom lens system according to the present Embodiment I-4, thethird lens element L3 and the fourth lens element L4 are composed of aglass material, while the fifth lens element L5 is composed of a resinmaterial such as an acrylic resin. Further, in the third lens elementL3, the object side surface (surface 6) is aspheric. In the fifth lenselement L5, the object side surface (surface 9) is aspheric.

Further, in the zoom lens system according to Embodiment I-4, the thirdlens unit G3 comprises solely a bi-convex sixth lens element L6. Thesixth lens element L6 is composed of a glass material. Its two surfaces(surfaces 11 and 12) are aspheric.

As such, in the zoom lens system according to Embodiments I-1 to I-4,the lens units G1 to G3 are arranged in a desired optical powerconstruction so that size reduction is achieved in the entire lenssystem in a state that excellent optical, performance is satisfied.

Here, in the zoom lens system according to Embodiments I-1 to I-4, thefirst lens unit G1 is composed of two lens elements, the second lensunit G2 is composed of three lens elements, and the third lens unit G3is composed of one lens element. As such, the zoom lens system accordingto Embodiments I-1 to I-4 has a small number of lens elementsconstituting each lens unit and a small thickness of each lens unit thatdirectly affects the thickness of the entire lens system. This permitsreduction in the thickness especially at the time of retraction.

Further, in a zoom lens system like that of Embodiments I-1 to I-4 that,in order from the object side to the image side, comprises: a first lensunit having negative optical power; a second lens unit that has positiveoptical power and that contains a bi-convex lens element composed of aresin material and serving as the most image side lens element; and athird lens unit having positive optical power, when during magnificationchange, the first to the third lens units are moved along the opticalaxis in such a manner that the intervals between the individual lensunits should vary, a zoom lens system having a variable magnificationratio of approximately 3 to 4 can be constructed compactly. Further,when the most image side lens element of the second lens unit isconstructed from a bi-convex lens element composed of a resin material,weight reduction of the zoom lens system and compensation of curvatureof field especially near a telephoto limit are achieved satisfactorily.

Conditions are described below that are preferable to be satisfied by azoom lens system like the zoom lens system according to Embodiments I-1to I-4, in order from the object side to the image side, comprising: afirst lens unit having negative optical power; a second lens unit thathas positive optical power and that contains a bi-convex lens elementcomposed of a resin material and serving as the most image side lenselement; and a third lens unit having positive optical power, whereinthe individual lens units move along the optical axis in such a mannerthat the intervals between the individual lens units should vary so thatvariable magnification is achieved. Here, a plurality of conditions tobe satisfied are set forth for the zoom lens system according to eachembodiment. A construction that satisfies all the conditions is mostdesirable for the zoom lens system. However, when an individualcondition is satisfied, a zoom lens system providing the correspondingeffect can be obtained.

For example, a zoom lens system like the zoom lens system according toEmbodiments I-1 to I-4 is characterized in that the following condition(I-1) is satisfied.

|(R _(2mi1) +R _(2mi2))/(R _(2mi1) −R _(2mi2))|<1.0  (I-1)

where,

R_(2mi1) is a radius of curvature on the object side of the most imageside lens element of the second lens unit, and

R_(2mi2) is a radius of curvature on the image side of the most imageside lens element of the second lens unit.

The condition (I-1) relates to the shape factor of the most image sidelens element of the second lens unit. When the condition (I-1) is notsatisfied, a difference in the optical power increases between bothsides of the most image side lens element. This causes a large curvatureof field on a surface having high optical power, and hence satisfactoryoverall performance cannot be ensured.

Here, when the following condition (I-1)′ is further satisfied,aberration caused by an error in assembling of the lens element isreduced. Thus, sensitivity of the performance degradation to the erroris reduced, and so is variation in the optical performance of the zoomlens system after assembling.

|(R _(2mi1) +R _(2mi2))/(R _(2mi1) −R _(2mi2))|<0.5  (I-1)′

Further, in a zoom lens system like the zoom lens system according toEmbodiments I-1 to I-4, it is preferable that the following condition(I-2) is satisfied.

(T _(G1) +T _(G2) +T _(G3))/f _(W)<2.7  (I-2)

(here, ω_(W)>30 and 3.0<f_(T)/f_(W)<4.0)

where,

T_(G1) is an optical axial thickness of the first lens unit,

T_(G2) is an optical axial thickness of the second lens unit,

T_(G3) is an optical axial thickness of the third lens unit,

ω_(W) is a half view angle at a wide-angle limit,

f_(W) is a focal length of the entire system at a wide-angle limit, and

f_(T) is a focal length of the entire system at a telephoto limit.

The condition (I-2) relates to the optical axial thicknesses of thefirst to the third lens units at the time of retraction. For the purposeof reducing the length of a lens barrel at the time of retraction, it ismost effective to reduce the optical axial length (thickness) of eachlens unit. When the thickness of each lens unit is set up such as tosatisfy the condition (I-2), the lens barrel total length at the time ofretraction is reduced further. This provides a zoom lens system moresuitable for a compact stylish camera.

When the following condition (I-2)′ is further satisfied, theabove-mentioned effect achieved more successfully.

(T _(G1) +T _(G2) +T _(G3))/f_(W)<2.3  (I-2)′

(here, ω_(W)>30 and 3.0<f_(T)/f_(W)<4.0)

In a zoom lens system like the zoom lens system according to EmbodimentsI-1 to I-4 in which the first lens unit, in order from the object sideto the image side, comprises a first lens element having negativeoptical power and a second lens element having positive optical power,it is preferable that the following condition (I-3) is satisfied.

0.3<T ₁ /f _(W)<1.3  (I-3)

(here, ω_(W)>30 and 3.0<f_(T)/f_(W)<4.0)

where,

T₁ is an air space between the first lens element and the second lenselement,

ω_(W) is a half view angle at a wide-angle limit,

f_(W) is a focal length of the entire system at a wide-angle limit, and

f_(T) is a focal length of the entire system at a telephoto limit.

When the first lens unit comprises, in order from the object side to theimage side, a first lens element having negative optical power and asecond lens element having positive optical power, chromatic aberrationand astigmatism especially at a wide-angle limit can be removedsatisfactorily. When the condition (I-3) is further satisfied, a zoomlens system is realized that is provided with improved compactness andmore satisfactorily compensated performance.

When the value goes below the lower limit of the condition (I-3), theair space between the first lens element and the second lens elementbecomes excessively small. This easily causes geometrical interferencebetween the first lens element rear face whose optical power has beenincreased for the purpose of ensuring the view angle and the front faceof the second lens element. This causes difficulty in ensuring therequired image surface illuminance. In contrast, when the value exceedsthe upper limit of the condition (I-3), the interval of the first lensunit increases, and hence a possibility arises that the compactness atthe time of retraction degrades.

When the following condition (I-3)′ is further satisfied, theabove-mentioned effect is achieved more successfully.

T ₁ /f _(W)<0.7  (I-3)′

(here, ω_(W)>30 and 3.0<f_(T)/f_(W)<4.0)

In a zoom lens system like the zoom lens system according to EmbodimentsI-1 to I-4 in which the first lens unit, in order from the object sideto the image side, comprises a first lens element having negativeoptical power and a second lens element having positive optical power,it is preferable that the following condition (I-4) is satisfied.

−4.2<(R ₁₂ +R ₂₁)/(R ₁₂ −R ₂₁)<−3.2  (I-4)

where,

R₁₂ is a radius of curvature on the image side of the first lenselement, and

R₂₁ is a radius of curvature on the object side of the second lenselement.

When the first lens unit comprises, in order from the object side to theimage side, a first lens element having negative optical power and asecond lens element having positive optical power, chromatic aberrationand astigmatism especially at a wide-angle limit can be removedsatisfactorily as described above. When the condition (I-4) is furthersatisfied, a zoom lens system is realized that has more satisfactorilyoverall performance.

When the value goes below the lower limit of the condition (I-4), theradius of curvature on the image side of the first lens element becomesshorter. This causes difficulty in maintaining the negative opticalpower required in the first lens unit, and hence causes a possibilitythat the compactness of the zoom lens system degrades. In contrast, whenthe value exceeds the upper limit of the condition (I-4), the radius ofcurvature on the image side of the first lens element becomes longer.This causes difficulty in reducing negative distortion.

Here, when at least one of the following conditions (I-4)′ and (I-4)″ isfurther satisfied, the above-mentioned effect is achieved moresuccessfully.

−4.0<(R ₁₂ +R ₂₁)/(R ₁₂ −R ₂₁)  (I-4)′

(R ₁₂ +R ₂₁)/(R ₁₂ −R ₂₁)<−3.5  (I-4)″

In a zoom lens system like the zoom lens system according to EmbodimentsI-1 to I-4 in which the first lens unit, in order from the object sideto the image side, comprises a first lens element having negativeoptical power and a second lens element having positive optical power,it is preferable that the following condition (I-5) is satisfied.

nd₂>1.88  (I-5)

where,

nd₂ is a refractive index of the second lens element to the d-line.

When the first lens unit comprises, in order from the object side to theimage side, a first lens element having negative optical power and asecond lens element having positive optical power, chromatic aberrationand astigmatism especially at a wide-angle limit can be removedsatisfactorily as described above. When the condition (I-5) is furthersatisfied, the curvature of each surface can be weaker. This permitseasier compensation of curvature of field especially at a wide-anglelimit. At the same time, the lens center thickness can be reduced, andhence further size reduction is achieved.

When the following condition (I-5)′ is further satisfied, theabove-mentioned effect is achieved more successfully.

nd₂>1.90  (I-5)′

Further, when the following condition (I-5)″ is satisfied, the opticalpower of the first lens unit can be increased. This permits constructingof a compact zoom lens system having a higher variable magnificationratio and higher optical performance.

2.50>nd₂  (I-5)″

In a zoom lens system like the zoom lens system according to EmbodimentsI-1 to I-4 in which the second lens unit, in order from the object sideto the image side, comprises a lens element A composed of a glassmaterial, a lens element B composed of a glass material and a lenselement C composed of a resin material, it is preferable that thefollowing condition (I-6) is satisfied.

2.0<φ_(A)/φ_(C)<3.5  (I-6)

where,

φ_(A) is a paraxial optical power of the lens element A, and

φ_(C) is a paraxial optical power of the lens element C.

When the second lens unit, in order from the object side to the imageside, comprises a lens element A composed of a glass material, a lenselement B composed of a glass material and a lens element C composed ofa resin material, a zoom lens system is realized that is provided withimproved compactness and satisfactorily compensated performance.Further, when the number of lens elements constituting the second lensunit is reduced as much as possible, an effect is obtained that thethickness especially at the time of retraction is reduced. Moreover,when one of these lens elements is composed of a resin material, furtherweight reduction and cost reduction can be achieved simultaneously.

When the value goes below the lower limit of the condition (I-6), theoptical power of the lens element C becomes excessively high within thesecond lens unit. Thus, the aberration to be compensated in the lenselement C increases. This causes difficulty in forming of the lenselement C. Further, the sensitivity of performance degradation to anassembling error becomes high. This causes difficulty in assembling. Incontrast, when the value exceeds the upper limit of the condition (I-6),the optical power of the lens element A increases. Thus, in order thatthe optical performance should be maintained satisfactorily, thethickness of the lens element A need be increased. This degrades thecompactness. Further, when the thickness of the lens element A is to bemaintained small, the aberration generated in the lens element A cannotsufficiently be compensated, and hence high optical performance cannotbe maintained.

Here, when at least one of the following conditions (I-6)′ and (I-6)″ isfurther satisfied, the above-mentioned effect is achieved moresuccessfully.

2.3<φ_(A)/φ_(C)  (I-6)′

φ_(A)/φ_(C)<3.2  (I-6)″

In a zoom lens system like the zoom lens system according to EmbodimentsI-1 to I-4 in which the second lens unit, in order from the object sideto the image side, comprises a lens element A composed of a glassmaterial, a lens element B composed of a glass material and a lenselement C composed of a resin material and in which each of the lenselement A and the lens element C has an aspheric surface, it ispreferable that the following condition (I-7) is satisfied.

0.15<φ_(Aa)/φ_(Ca)<0.30  (I-7)

where,

φ_(Aa) is a paraxial optical power of the aspheric surface of the lenselement A, and

φ_(Ca) is a paraxial optical power of the aspheric surface of the lenselement C.

When the second lens unit, in order from the object side to the imageside, comprises a lens element A composed of a glass material, a lenselement B composed of a glass material and a lens element C composed ofa resin material, a zoom lens system is realized that is provided withimproved compactness and satisfactorily compensated performance asdescribed above. Further, when the number of lens elements constitutingthe second lens unit is reduced as much as possible, an effect isobtained that the thickness especially at the time of retraction isreduced. Moreover, when one of these lens elements is composed of aresin material, further weight reduction and cost reduction can beachieved simultaneously. Further, each of the lens element A located onthe object side among the two lens elements composed of a glass materialand the lens element C composed of a resin material has an asphericsurface. By virtue of this, the image surface position can be alignedfor every view angle. Thus, more satisfactory optical performance isobtained.

When the value goes below the lower limit of the condition (I-7), theoptical power of the lens element C becomes excessively high within thesecond lens unit. Thus, the aberration to be compensated by the asphericsurface of the lens element C increases. This causes difficulty informing of the lens element C. Further, the sensitivity of performancedegradation to an assembling error becomes high. This causes difficultyin assembling. In contrast, when the value exceeds the upper limit ofthe condition (I-7), the optical power of the lens element A increases.Thus, in order that the optical performance should be maintainedsatisfactorily, the thickness of the lens element A need be increased.This degrades the compactness. Further, when the thickness of the lenselement A is to be maintained small, the aberration generated in thelens element A cannot sufficiently be compensated, and hence highoptical performance cannot be maintained.

Here, when at least one of the following conditions (I-7)′ and (I-7)″ isfurther satisfied, the above-mentioned effect is achieved moresuccessfully.

0.19<φ_(Aa)/φ_(Ca)  (I-7)′

φ_(Aa)/φ_(Ca)<0.25  (I-7)″

The conditions (I-6) and (I-7) may be satisfied individually orsimultaneously. When any one of the conditions is satisfied, a zoom lenssystem is realized that is provided with improved compactness andsatisfactorily compensated performance. Here, when the two conditionsare satisfied simultaneously, the effect is achieved more successfully.

The lens units constituting the zoom lens system of Embodiments I-1 toI-4 are composed exclusively of refractive type lens elements thatdeflect the incident light by refraction (that is, lens elements of atype in which deflection is achieved at the interface between media eachhaving a distinct refractive index). However, the present invention isnot limited to the zoom lens system of this construction. For example,the lens units may employ diffractive type lens elements that deflectthe incident light by diffraction; refractive-diffractive hybrid typelens elements that deflect the incident light by a combination ofdiffraction and refraction; or gradient index type lens elements thatdeflect the incident light by distribution of refractive index in themedium.

Further, in the zoom lens system according to Embodiments I-1 to I-4,the third lens unit has been composed of one lens element. However, aslong as the third lens unit has positive optical power, the number ofconstituting lens elements is not limited to a particular value.However, from the perspective of thickness reduction of the entire lenssystem, it is more preferable that the third lens unit is composed ofone lens element and that the entire zoom lens system is composed of sixlens elements at minimum. In particular, when the third lens unit isconstructed from one lens element composed of a glass material, a zoomlens system can be realized that has more satisfactory focusingperformance.

Further, in the zoom lens system according to Embodiments I-1 to I-4, areflecting surface may be arranged in the optical path so that theoptical path may be bent before or after the zoom lens system oralternatively in the middle. The bending position may be set uparbitrarily depending on the necessity. When the optical path is bentappropriately, thickness reduction in appearance can be achieved in acamera.

As described above, according to the present Embodiments I-1 to I-4, azoom lens system can be obtained that has high optical performancecompatible to an image sensor of high pixel number and that has areduced overall length and a reduced thickness.

Embodiments II-1 to II-4

FIG. 9 is a lens configuration diagram of a zoom lens system accordingto Embodiment II-1. FIG. 11 is a lens configuration diagram of a zoomlens system according to Embodiment II-2. FIG. 13 is a lensconfiguration diagram of a zoom lens system according to EmbodimentII-3. FIG. 15 is a lens configuration diagram of a zoom lens systemaccording to Embodiment II-4. Each configuration diagram shows a zoomlens system in an infinity in-focus condition. In each diagram, part (a)shows a lens configuration at a wide-angle limit (in the minimum focallength condition: focal length f_(W)). Part (b) shows a lensconfiguration at an approximately middle position (in an intermediatefocal length condition: focal length f_(M)=√{square root over ()}(f_(W)*f_(T))). Part (c) shows a lens configuration at a telephotolimit (in the maximum focal length condition: focal length f_(T)).

Each zoom lens system according to Embodiments II-1 to II-4, in orderfrom the object side to the image side, comprises: a first lens unit G1having negative optical power; a diaphragm A; a second lens unit G2having positive optical power; and a third lens unit G3 having positiveoptical power. In the zoom lens system according to Embodiments II-1 toII-4, in zooming from a wide-angle limit to a telephoto limit, the firstlens unit G1 moves with locus of a convex to the image side, while thesecond lens unit G2 and the diaphragm A monotonically move to the objectside, and while the third lens unit G3 moves with changing the intervalwith the second lens unit G2. That is, in the zoom lens system accordingto Embodiments II-1 to II-4, in zooming from a wide-angle limit to atelephoto limit, the lens units move respectively along the optical axisin such a manner that the interval between the first lens unit G1 andthe second lens unit G2 should decrease while the interval between thesecond lens unit G2 and the third lens unit G3 should vary.

In FIGS. 9, 11, 13 and 15, the straight line located on the mostright-hand side indicates the position of the image surface S of animage sensor such as a CCD. On the object side relative to this, a plateP is provided that is composed of a parallel plate or a cover glassplate equivalent to an optical low-pass filter or a face plate of animage sensor or the like.

As shown in FIG. 9, in the zoom lens system according to EmbodimentII-1, the first lens unit G1, in order from the object side to the imageside, comprises two lens elements consisting of: a negative meniscusobject side lens element L1 having negative optical power and with theconvex surface facing the object side; and a positive meniscus imageside lens element L2 having positive optical power and with the convexsurface facing the object side. In each of the object side lens elementL1 and the image side lens element L2, the image side surface isaspheric.

In the zoom lens system according to Embodiment II-1, the second lensunit G2, in order from the object side to the image side, comprises: abi-convex third lens element L3; a bi-concave fourth lens element L4;and a bi-convex fifth lens element L5. Among these, the third lenselement L3 and the fourth lens element L4 are cemented with each otherso as to constitute a cemented lens element. Further, the third lenselement L3 serving as the most object side lens element of the secondlens unit G2 has an aspheric object side surface. Further, the fifthlens element L5 serving as the most image side lens element of thesecond lens unit G2 is a lens element that has an aspheric object sidesurface and that is composed of the resin (ZEONEX (registered trademark)manufactured by ZEON CORPORATION).

In the zoom lens system according to Embodiment II-1, the third lensunit G3 comprises solely a bi-convex sixth lens element L6. In the sixthlens element L6, the image side surface is aspheric.

In the zoom lens system according to Embodiment II-1, the fifth lenselement L5 serving as the most image side lens element of the secondlens unit G2 is a lens element composed of a resin and having anaspheric surface. Further, as shown later in Table II-13, the most imageside lens element L5 of the second lens unit G2 is set to have a longfocal length and hence a low optical power such that the lens element ismade thin. By virtue of this, the second lens unit G2 serving as amoving unit is made into light weight. Further, since the optical poweris made low so that the influence of assembling error to image qualitydegradation is suppressed, cemented construction between the third lenselement L3 and the fourth lens element L4 is permitted so that thethickness of the second lens unit G2 is reduced. As a result, the zoomlens system according to Embodiment II-1 has a reduced weight and areduced overall optical length at the time of non-use.

As shown in FIG. 11, in the zoom lens system according to EmbodimentII-2, the first lens unit G1, in order from the object side to the imageside, comprises two lens elements consisting of: a negative meniscusobject side lens element L1 having negative optical power and with theconvex surface facing the object side; and a positive meniscus imageside lens element L2 having positive optical power and with the convexsurface facing the object side. In the object side lens element L1, theimage side surface is aspheric. In the image side lens element L2, theobject side surface is aspheric.

In the zoom lens system according to Embodiment II-2, the second lensunit G2, in order from the object side to the image side, comprises: apositive meniscus third lens element L3 with the convex surface facingthe object side; a bi-convex fourth lens element L4; a bi-concave fifthlens element L5; and a bi-convex sixth lens element L6. Among these, thefourth lens element L4 and the fifth lens element L5 are cemented witheach other so as to constitute a cemented lens element. Further, thethird lens element L3 serving as the most object side lens element ofthe second lens unit G2 has an aspheric object side surface. Further,the sixth lens element L6 serving as the most image side lens element ofthe second lens unit G2 is a lens element that has an aspheric objectside surface and that is composed of the resin (ZEONEX (registeredtrademark) manufactured by ZEON CORPORATION).

In the zoom lens system according to Embodiment II-2, the third lensunit G3 comprises solely a bi-convex seventh lens element L7. In theseventh lens element L7, the image side surface is aspheric.

In the zoom lens system according to Embodiment II-2, the sixth lenselement L6 serving as the most image side lens element of the secondlens unit G2 is a lens element composed of a resin and having anaspheric surface. Further, as shown later in Table II-13, the most imagesides lens element L6 of the second lens unit G2 is set to have a longfocal length and hence a low optical power such that the lens element ismade thin. By virtue of this, the second lens unit G2 serving as amoving unit is made into light weight. Further, since the optical poweris made low so that the influence of assembling error to image qualitydegradation is suppressed, cemented construction between the fourth lenselement L4 and the fifth lens element L5 is permitted so that thethickness of the second lens unit G2 is reduced. As a result, the zoomlens system according to Embodiment II-2 has a reduced weight and areduced overall optical length at the time of non-use.

As shown in FIG. 13, in the zoom lens system according to EmbodimentII-3, the first lens unit G1, in order from the object side to the imageside, comprises two lens elements consisting of: a negative meniscusobject side lens element L1 having negative optical power and with theconvex surface facing the object side; and a positive meniscus imageside lens element L2 having positive optical power and with the convexsurface facing the object side. In the object side lens element L1, theimage side surface is aspheric. In the image side lens element L2, theobject side surface is aspheric.

In the zoom lens system according to Embodiment II-3, the second lensunit G2, in order from the object side to the image side, comprises: abi-convex third lens element L3; a bi-concave fourth lens element L4;and a bi-convex fifths-lens element L5. Among these, the third lenselement L3 and the fourth lens element L4 are cemented with each otherso as to constitute a cemented lens element. Further, the third lenselement L3 serving as the most object side lens element of the secondlens unit G2 has an aspheric object side surface. Further, the fifthlens element L5 serving as the most image side lens element of thesecond lens unit G2 is a lens element that has an aspheric object sidesurface and that is composed of the resin (ZEONEX (registered trademark)manufactured by ZEON CORPORATION).

In the zoom lens system according to Embodiment II-3, the third lensunit G3 comprises solely a bi-convex sixth lens element L6. In the sixthlens element L6, two surfaces are aspheric.

In the zoom lens system according to Embodiment II-3, the fifth lenselement L5 serving as the most image side lens element of the secondlens unit G2 is a lens element composed of a resin and having anaspheric surface. Further, as shown later in Table II-13, the most imageside lens element L5 of the second lens unit G2 is set to have a longfocal length and hence a low optical power such that the lens element ismade thin. By virtue of this, the second lens unit G2 serving as amoving unit is made into light weight. Further, since the optical poweris made low so that the influence of assembling error to image qualitydegradation is suppressed, cemented construction between the third lenselement L3 and the fourth lens element L4 is permitted so that thethickness of the second lens unit G2 is reduced. As a result, the zoomlens system according to Embodiment II-3 has a reduced weight and areduced overall optical length at the time of non-use.

As shown in FIG. 15, in the zoom lens system according to EmbodimentII-4, the first lens unit G1, in order from the object side to the imageside, comprises two lens elements consisting of: a negative meniscusobject side lens element L1 having negative optical power and with theconvex surface facing the object side; and a positive meniscus imageside lens element L2 having positive optical power and with the convexsurface facing the object side. In the object side lens element L1, theimage side surface is aspheric. In the image side lens element L2, theobject side surface is aspheric.

In the zoom lens system according to Embodiment II-4, the second lensunit G2, in order from the object side to the image side, comprises: apositive meniscus third lens element L3 with the convex surface facingthe object side; a bi-convex fourth lens element L4; a bi-concave fifthlens element L5; and a bi-convex sixth lens element L6. Among these, thefourth lens element L4 and the fifth lens element L5 are cemented witheach other so as to constitute a cemented lens element. Further, thethird lens element L3 serving as the most object side lens element ofthe second lens unit G2 has an aspheric object side surface. Further,the sixth lens element L6 serving as the most image side lens element ofthe second lens unit G2 is a lens element that has an aspheric objectside surface and that is composed of the resin (ZEONEX (registeredtrademark) manufactured by ZEON CORPORATION).

In the zoom lens system according to Embodiment II-4, the third lensunit G3 comprises solely a bi-convex seventh lens element L7. In theseventh lens element L7, two surfaces are aspheric.

In the zoom lens system according to Embodiment II-4, the sixth lenselement L6 serving as the most image side lens element of the secondlens unit G2 is a lens element composed of a resin and having anaspheric surface. Further, as shown later in Table II-13, the most imageside lens element L6 of the second lens unit G2 is set to have a longfocal length and hence a low optical power such that the lens element ismade thin. By virtue of this, the second lens unit G2 serving as amoving unit is made into light weight. Further, since the optical poweris made low so that the influence of assembling error to image qualitydegradation is suppressed, cemented construction between the fourth lenselement L4 and the fifth lens element L5 is permitted so that thethickness of the second lens unit G2 is reduced. As a result, the zoomlens system according to Embodiment II-4 has a reduced weight and areduced overall optical length at the time of non-use.

As such, in the zoom lens system according to Embodiments II-1 to II-4,the lens units G1 to G3 are arranged in a desired optical powerconstruction so that size reduction is achieved in the entire lenssystem in a state that excellent optical performance is satisfied.

In particular, in the zoom lens system according to Embodiments II-1 toII-4, the first lens unit G1 is composed of one object side lens elementhaving negative optical power and one image side lens element havingpositive optical power. Further, the second lens unit G2 contains acemented lens element composed of a positive lens element and a negativelens element, while the third lens unit G3 is composed of one lenselement. As such, the zoom lens system according to Embodiments II-1 toII-4 realizes a lens system that has a small number of lens elementsconstituting each lens unit and a reduced overall optical length at thetime of non-use.

Further, in particular, in the zoom lens system according to EmbodimentsII-1 to II-4, a lens element composed of a resin and having at least oneaspheric surface is employed as the most image side lens element of thesecond lens unit G2. This permits weight reduction of the entire lenssystem in a state that the excellent optical performance is maintained.Further, in such a lens element composed of a resin, a manufacturingerror is reduced, and hence mass-production can be performed relativelyeasily. Here, the lens element composed of a resin employed in the mostimage side lens element is not limited to particular one, as long asbeing in accordance with the object of the present invention. Forexample, a lens element may be employed that is formed with a syntheticresin such as polymethylmethacrylate, polycarbonate, polystyrene andstyrene-acrylonitrile copolymer generally employed as the material of anoptical lens into a shape where at least one surface is aspheric.

Further, in the zoom lens system according to Embodiments II-1 to II-4,one bi-convex lens element constituting the third lens unit G3 has atleast one aspheric surface. By virtue of this, in the zoom lens systemaccording to Embodiments II-1 to II-4, in particular, the amount ofdeviation in the image formation position at every image height can bereduced remarkably.

Conditions are described below that are to be satisfied by a zoom lenssystem like the zoom lens system according to Embodiments II-1 to II-4that forms an optical image of an object with variable magnification andthat, in order from the object side to the image side, comprises a firstlens unit having negative optical power, a second lens unit havingpositive optical power, and a third lens unit having positive opticalpower, wherein the first lens unit is composed of one object side lenselement and one image side lens element having positive optical power,and wherein the most image side lens element constituting the secondlens unit is a lens element composed of a resin and having at least oneaspheric surface. Here, a plurality of conditions to be satisfied areset forth for the zoom lens system according to each embodiment. Aconstruction that satisfies all the conditions is most desirable for thezoom lens system. However, when an individual condition is satisfied, azoom lens system providing the corresponding effect can be obtained.

For example, a zoom lens system like the zoom lens system according toEmbodiments II-1 to II-4 is characterized in that the followingcondition (II-1) is satisfied.

0.7<f ₂ /f _(2r)<1.5  (II-1)

where,

f₂ is a composite focal length of the second lens unit, and

f_(2r) is a focal length of the most image side lens element of thesecond lens unit.

The condition (II-1) sets forth the ratio between the focal length ofthe entire second lens unit and the focal length of the most image sidelens element of the second lens unit so as to avoid a situation that theaberration sensitivity of the most image side lens element in the secondlens unit increases excessively. When the value goes below the lowerlimit of the condition (II-1), the optical power of the second lens unitdecreases. Thus, in order that a zoom ratio of 3 or greater should beachieved, the moving distance of the second lens unit need be increased.This causes difficulty in size reduction. In contrast, when the valueexceeds the upper limit of the condition (II-1), the optical power ofthe second lens unit increases. This causes difficulty in satisfactorycompensation of the aberration generated in the second lens unit, whichis to be performed by the first lens unit and the third lens unit.

Here, when at least one of the following conditions (II-1)′ and (II-1)″is further satisfied, the above-mentioned effect is achieved moresuccessfully.

0.8<f ₂ /f _(2r)  (II-1)′

f ₂ /f _(2r)<1.1  (II-1)″

Further, in a zoom lens system like the zoom lens system according toEmbodiments II-1 to II-4, it is preferable that the following condition(II-2) is satisfied.

0.2<f _(W) /f _(2r)<0.6  (II-2)

(here, ω_(W)>37)

where,

f_(W) is a focal length of the entire system at wide-angle limit,

f_(2r) is a focal length of the most image side lens element of thesecond lens unit, and

ω_(W) is a half view angle at a wide-angle limit.

The condition (II-2) sets forth an appropriate focal length of the mostimage side lens element of the second lens unit. When the condition(II-2) is satisfied, a situation is avoided that the aberrationsensitivity of the most image side lens element in all the lens unitsincreases excessively.

Here, when at least one of the following conditions (II-2)′ and (II-2)″is further satisfied, the above-mentioned effect is achieved moresuccessfully.

0.35<f _(W) /f _(2r)  (II-2)′

f _(W) /f ₂<0.45  (II-2)″

(here, ω_(W)>37)

Further, in a zoom lens system like the zoom lens system according toEmbodiments II-1 to II-4, it is preferable that the following condition(II-3) is satisfied.

T2/Y>0.8  (II-3)

where,

T2 is a center thickness of the second lens unit, and

Y is a maximum image height.

The condition (II-3) sets forth the center thickness of the second lensunit. When the condition (II-3) is satisfied, a situation is avoidedthat the thickness becomes excessively small and hence that satisfactoryaberration compensation cannot be achieved.

Here, when the following condition (II-3)′ is further satisfied, theabove-mentioned effect is achieved more successfully. Further, when thefollowing condition (II-3)″ is satisfied, a possibility is avoided thatthe thickness of the second lens unit increases excessively and so doesthe overall optical length at the time of non-use.

T2/Y>0.83  (II-3)′

1.10>T2/Y  (II-3)″

Further, in a zoom lens system like the zoom lens system according toEmbodiments II-1 to II-4 in which the second lens unit has at least acemented lens element composed of a positive lens element and a negativelens element, it is preferable that the following conditions (II-4) and(II-5) are satisfied.

−20<Rc/Y<−1  (II-4)

2.0<(T1+T2+T3)/Y<3.5  (II-5)

where,

Rc is a radius of curvature of a cemented surface of the cemented lenselement in the second lens unit,

Y is a maximum image height,

T1 is a center thickness of the first lens unit,

T2 is a center thickness of the second lens unit, and

T3 is a center thickness of the third lens unit.

The condition (II-4) sets forth the radius of curvature of the cementedsurface in the cemented lens element constituting the second lens unit.The condition (II-5) sets forth the total center thickness of the lensunits. When these conditions (II-4) and (II-5) are satisfied, asituation is avoided that the optical power of the lens element locatedon the most object side of the second lens unit increases excessively.Further, a possibility is avoided that the total thickness of the lensunits increases excessively and so does the overall optical length atthe time of non-use.

Here, when at least one of the following conditions (II-4)′, (II-4)″,(II-5)′ and (II-5)″ is further satisfied, the above-mentioned effect isachieved more successfully.

−12<Rc/Y  (II-4)′

Rc/Y<−2  (II-4)″

2.3<(T1+T2+T3)/Y  (II-5)′

(T1+T2+T3)/Y<3.1  (II-5)″

Further, in a zoom lens system like the zoom lens system according toEmbodiments II-1 to II-4 in which the third lens unit is composed of onebi-convex lens element having at least one aspheric surface, it ispreferable that the following conditions (II-6) and (II-7) aresatisfied.

f _(3r) /f _(T)<1.5  (II-6)

T3/Y<0.8  (II-7)

(here, ω_(T)<15)

where,

f_(3r) is a focal length of the bi-convex lens element of the third lensunit,

f_(T) is a focal length of the entire system at a telephoto limit,

T3 is a center thickness of the third lens unit,

Y is a maximum image height, and

ω_(T) is a half view angle at a telephoto limit.

The condition (II-6) sets forth an appropriate focal length of thebi-convex lens element constituting, the third lens unit. Further, thecondition (II-7) sets forth the center thickness of the third lens unit.When these conditions (II-6) and (II-7) are satisfied, the aberrationsensitivity of the third lens unit is reduced relative to all the lensunits. This avoids easy occurrence of performance degradation caused bythe movement of the third lens unit at each zooming position. Further, apossibility is avoided that the thickness of the third lens unitincreases excessively and so does the overall optical length at the timeof non-use.

Here, when at least one of the following conditions (II-6)′ and (II-7)′is further satisfied, the above-mentioned effect is achieved moresuccessfully. Moreover, when at least one of the following conditions(II-6)″ and (II-7)″ is satisfied, a situation is avoided that the movingdistance of the third lens unit increases excessively during focusing.

f _(3r) /f _(T)<1.3  (II-6)′

1.0<f _(3r) /f _(T)  (II-6)″

T3/Y<0.5  (II-7)′

0.2<T3/Y  (II-7)″

(here, ω_(T)<15)

Here, the lens units constituting the zoom lens system of EmbodimentsII-1 to II-4 are composed exclusively of refractive type lens elementsthat deflect the incident light by refraction (that is, lens elements ofa type in which deflection is achieved at the interface between mediaeach having a distinct refractive index). However, the present inventionis not limited to the zoom lens system of this construction. Forexample, the lens units may employ diffractive type lens elements thatdeflect the incident light by diffraction; refractive-diffractive hybridtype lens elements that deflect the incident light by a combination ofdiffraction and refraction; or gradient index type lens elements thatdeflect the incident light by distribution of refractive index in themedium.

Further, in the zoom lens system according to Embodiments II-1 to II-4,a reflecting surface may be arranged in the optical path so that theoptical path may be bent before or after the zoom lens system oralternatively in the middle. The bending position may be set uparbitrarily depending on the necessity. When the optical path is bentappropriately, thickness reduction in appearance can be achieved in acamera.

Further, the zoom lens system according to Embodiments II-1 to II-4 hasbeen described for the construction that a plate P equivalent to anoptical low-pass filter is arranged between the most image side surfaceof the third lens element G3 and the image surface S. This low-passfilter may be a birefringent type low-pass filter made of, for example,a crystal whose predetermined crystal orientation is adjusted; or aphase type low-pass filter that achieves required characteristics ofoptical cut-off frequency by diffraction. Further, this plate P may bearranged depending on the necessity.

As described above, according to the present Embodiments II-1 to II-4, azoom lens system can be obtained in which curvature of field iscompensated satisfactorily and which has a reduced weight and a reducedoverall optical length at the time of non-use.

Embodiments III-1 to III-4

FIG. 17 is a lens configuration diagram of a zoom lens system accordingto Embodiment III-1. FIG. 19 is a lens configuration diagram of a zoomlens system according to Embodiment III-2. FIG. 21 is a lensconfiguration diagram of a zoom lens system according to EmbodimentIII-3. FIG. 23 is a lens configuration diagram of a zoom lens systemaccording to Embodiment III-4. Each configuration diagram shows a zoomlens system in an infinity in-focus condition. In each diagram, part (a)shows a lens configuration at a wide-angle limit (in the minimum focallength condition: focal length f_(W)). Part (b) shows a lensconfiguration at an approximately middle position (in an intermediatefocal length condition: focal length f_(M)=√{square root over ()}(f_(W)*f_(T))). Part (c) shows a lens configuration at a telephotolimit (in the maximum focal length condition: focal length f_(T)).

Each zoom lens system according to Embodiments III-1 to III-4, in orderfrom the object side to the image side, comprises: a first lens unit G1having negative optical power; a diaphragm A; a second lens unit G2having positive optical power; and a third lens unit G3 having positiveoptical power. In the zoom lens system according to Embodiments III-1 toIII-4, in zooming from a wide-angle limit to a telephoto limit, thefirst lens unit G1 moves with locus of a convex to the image side, whilethe second lens unit G2 and the diaphragm A monotonically move to theobject side, and while the third lens unit G3 moves with changing theinterval with the second lens unit G2. That is, in the zoom lens systemaccording to Embodiments III-1 to III-4, in zooming from a wide-anglelimit to a telephoto limit, the lens units move respectively along theoptical axis in such a manner that the interval between the first lensunit G1 and the second lens unit G2 should decrease while the intervalbetween the second lens unit G2 and the third lens unit G3 should vary.

In FIGS. 17, 19, 21 and 23, the straight line located on the mostright-hand side indicates the position of the image surface S of animage sensor such as a CCD. On the object side relative to this, a plateP is provided that is composed of a parallel plate or a cover glassplate equivalent to an optical low-pass filter or a face plate of animage sensor or the like.

As shown in FIG. 17, in the zoom lens system according to EmbodimentIII-1, the first lens unit G1, in order from the object side to theimage side, comprises two lens elements consisting of: a negativemeniscus object side lens element L1 having negative optical power andwith the convex surface facing the object side; and a positive meniscusimage side lens element L2 having positive optical power and with theconvex surface facing the object side. In each of the object side lenselement L1 and the image side lens element L2, the image side surface isaspheric.

In the zoom lens system according to Embodiment III-1, the second lensunit G2, in order from the object side to the image side, comprises: apositive meniscus third lens element L3 with the convex surface facingthe object side; a negative meniscus fourth lens element L4 with theconvex surface facing the object side; and a bi-convex fifth lenselement L5. Among these, the third lens element L3 and the fourth lenselement L4 are cemented with each other so as to constitute a positivecemented lens element. Further, the third lens element L3 serving as themost object side lens element of the second lens unit G2 has an asphericobject side surface.

Further, in the zoom lens system according to Embodiment III-1, thethird lens unit G3 comprises solely a positive meniscus sixth lenselement L6 with the convex surface facing the image side. In the sixthlens element L6, the image side surface is aspheric.

In the zoom lens system according to Embodiment III-1, as shown later inTable III-13, the image side lens element L2 constituting the first lensunit G1 has a notably high refractive index. Thus, in the object sidelens element L1, the lens center thickness can be reduced in a statethat the thickness at large light ray height is easily ensured. Thus,the zoom lens system according to Embodiment III-1 has a reduced overalloptical length at the time of non-use.

As shown in FIG. 19, in the zoom lens system according to EmbodimentIII-2, the first lens unit G1, in order from the object side to theimage side, comprises two lens elements consisting of: a negativemeniscus object side lens element L1 having negative optical power andwith the convex surface facing the object side; and a positive meniscusimage side lens element L2 having positive optical power and with theconvex surface facing the object side. In the object side lens elementL1, the image side surface is aspheric. In the image side lens elementL2, the object side surface is aspheric.

In the zoom lens system according to Embodiment III-2, the second lensunit G2, in order from the object side to the image side, comprises: apositive meniscus third lens element L3 with the convex surface facingthe object side; a bi-convex fourth lens element L4; a bi-concave fifthlens element L5; and a bi-convex sixth lens element L6. Among these, thefourth lens element L4 and the fifth lens element L5 are cemented witheach other so as to constitute a cemented lens element. Further, each ofthe third lens element L3 serving as the most object side lens elementand the sixth lens element L6 serving as the most image side lenselement in the second lens unit G2 has an aspheric object side surface.

In the zoom lens system according to Embodiment III-2, the third lensunit G3 comprises solely a bi-convex seventh lens element L7. In theseventh lens element L7, the object side surface is aspheric.

In the zoom lens system according to Embodiment III-2, as shown later inTable III-13, the image side lens element L2 constituting the first lensunit G1 has a notably high refractive index. Thus, in the object sidelens element L1, the lens center thickness can be reduced in a statethat the thickness at large light ray height is easily ensured. Thus,the zoom lens system according to Embodiment III-2 has a reduced overalloptical length at the time of non-use.

As shown in FIG. 21, in the zoom lens system according to EmbodimentIII-3, the first lens unit G1, in order from the object side to theimage side, comprises two lens elements consisting of: a negativemeniscus object side lens element L1 having negative optical power andwith the convex surface facing the object side; and a positive meniscusimage side lens element L2 having positive optical power and with theconvex surface facing the object side. In the object side lens elementL1, the image side surface is aspheric. In the image side lens elementL2, the object side surface is aspheric.

Further, in the zoom lens system according to Embodiment III-3, thesecond lens unit G2, in order from the object side to the image side,comprises: a bi-convex third lens element L3; a bi-concave fourth lenselement L4; and a bi-convex fifth lens element L5. Among these, thethird lens element L3 and the fourth lens element L4 are cemented witheach other so as to constitute a positive cemented lens element.Further, each of the third lens element L3 serving as the most objectside lens element and the fifth lens element L5 serving as the mostimage side lens element of the second lens unit G2 has an asphericobject side surface.

In the zoom lens system according to Embodiment III-3, the third lensunit G3 comprises solely a bi-convex sixth lens element L6. In the sixthlens element L6, two surfaces are aspheric.

In the zoom lens system according to Embodiment III-3, as shown later inTable III-13, the image side lens element L2 constituting the first lensunit G1 has a notably high refractive index. Thus, in the object sidelens element L1, the lens center thickness can be reduced in a statethat the thickness at large light ray height is easily ensured. Thus,the zoom lens system according to Embodiment III-3 has a reduced overalloptical length at the time of non-use.

As shown in FIG. 23, in the zoom lens system according to EmbodimentIII-4, the first lens unit G1, in order from the object side to theimage side, comprises two lens elements consisting of: a negativemeniscus object side lens element L1 having negative optical power andwith the convex surface facing the object side; and a positive meniscusimage side lens element L2 having positive optical power and with theconvex surface facing the object side. In the object side lens elementL1, the image side surface is aspheric. In the image side lens elementL2, the object side surface is aspheric.

In the zoom lens system according to Embodiment III-4, the second lensunit G2, in order from the object side to the image side, comprises: apositive meniscus third lens element L3 with the convex surface facingthe object side; a bi-convex fourth lens element L4; a bi-concave fifthlens element L5; and a bi-convex sixth lens element L6. Among these, thefourth lens element L4 and the fifth lens element L5 are cemented witheach other so as to constitute a cemented lens element. Further, each ofthe third lens element L3 serving as the most object side lens elementand the sixth lens element L6 serving as the most image side lenselement in the second lens unit G2 has an aspheric object side surface.

In the zoom lens system according to Embodiment III-4, the third lensunit G3 comprises solely a bi-convex seventh lens element L7. In theseventh lens element L7, two surfaces are aspheric.

In the zoom lens system according to Embodiment III-4, as shown later inTable III-13, the image side lens element L2 constituting the first lensunit G1 has a notably high refractive index. Thus, in the object sidelens element L1, the lens center thickness can be reduced in a statethat the thickness at large light ray height is easily ensured. Thus,the zoom lens system according to Embodiment III-4 has a reduced overalloptical length at the time of non-use.

As such, in the zoom lens system according to Embodiments III-1 toIII-4, the lens units G1 to G3 are arranged in a desired optical powerconstruction so that size reduction is achieved in the entire lenssystem in a state that excellent optical performance is satisfied.

In particular, in the zoom lens system according to Embodiments III-1 toIII-4, the first lens unit G1 is composed of one object side lenselement having negative optical power and one image side lens elementhaving positive optical power. Further, the second lens unit G2, inorder from the object side to the image side, comprises one set ofpositive cemented lens element and one positive lens element, oralternatively has such a construction that one set of cemented lenselement is placed between positive lens elements each arranged on theobject side or the image side. Moreover, the third lens unit G3 iscomposed of one lens element. As such, the zoom lens system according toEmbodiments III-1 to III-4 realizes a lens system that has a smallnumber of lens elements constituting each lens unit and a reducedoverall optical length at the time of non-use.

Here, as described above, in the zoom lens system according toEmbodiments III-1 to III-4, the second lens unit G2 is composed of oneset of positive cemented lens element and one positive lens element oralternatively has such a construction that one set of cemented lenselement is placed between positive lens elements each arranged on theobject side or the image side. Instead, the second lens unit G2 maycomprise two sets of positive cemented lens elements each composed oftwo lens elements, so that a lens system can be realized that has areduced overall optical length at the time of non-use.

Further, in the zoom lens system according to Embodiments III-1 toIII-4, each of the object side lens element and the image side lenselement constituting the first lens unit G1 has an aspheric surface.Thus, the zoom lens system according to Embodiments III-1 to III-4 hasexcellent optical performance, for example, in compensation of curvatureof field.

Conditions are described below that are to be satisfied by a zoom lenssystem like the zoom lens system according to Embodiments III-1 to III-4that forms an optical image of an object with variable magnification ofa factor of 3 or greater and that, in order from the object side to theimage side, comprises a first lens unit having negative optical power, asecond lens unit having positive optical power, and a third lens unithaving positive optical power, wherein the first lens unit is composedof one object side lens element and one image side lens element, andwherein the object side lens element has negative optical power whilethe image side lens element has positive optical power. Here, aplurality of conditions to be satisfied are set forth for the zoom lenssystem according to each embodiment. A construction that satisfies allthe conditions is most desirable for the zoom lens system. However, whenan individual condition is satisfied, a zoom lens system providing thecorresponding effect can be obtained.

For example, a zoom lens system like the zoom lens system according toEmbodiments III-1 to III-4 is characterized in that the followingcondition (III-1) is satisfied.

1.5<f ₂ /f _(W)<2.8  (III-1)

(here, ω_(W)>36)

where,

f₂ is a composite focal length of the second lens unit,

f_(W) is a focal length of the entire system at a wide-angle limit, and

ω_(W) is a half view angle at a wide-angle limit.

The condition (III-1) sets forth an appropriate focal length of thesecond lens unit. When the value goes below the lower limit of thecondition (III-1), the optical power of the second lens unit decreases.Thus, in order that a zoom ratio of 3 or greater should be achieved, themoving distance of the second lens unit need be increased. This causesdifficulty in size reduction. In contrast, when the value exceeds theupper limit of the condition (III-1), the optical power of the secondlens unit increases. This causes difficulty in satisfactory compensationof the aberration generated in the second lens unit, which is to beperformed by the first lens unit and the third lens unit.

Here, when at least one of the following conditions (III-1)′ and(III-1)″ is further satisfied, the above-mentioned effect is achievedmore successfully.

2.0<f ₂ /f _(W)  (III-1)′

f ₂ /f _(W)<2.7  (III-1)″

(here, ω_(W)>36)

Further, in a zoom lens system like the zoom lens system according toEmbodiments III-1 to III-4, it is preferable that the followingconditions (III-2), (III-3), (III-4), (III-5) and (III-6) are satisfied.

n12>1.91  (III-2)

ν12<29  (III-3)

n11>1.50  (III-4)

ν11>35  (III-5)

n12−n11>0.10  (III-6)

where,

n12 is a refractive index of the image side lens element of the firstlens unit,

ν12 is an Abbe number of the image side lens element of the first lensunit,

n11 is a refractive index of the object side lens element of the firstlens unit, and

ν11 is an Abbe number of the object side lens element of the first lensunit.

The conditions (III-2) and (III-3) set forth the refractive index andthe Abbe number of the image side lens element constituting the firstlens unit. When these conditions (III-2) and (III-3) are satisfied, thecenter thickness of the image side lens element becomes small, whilecurvature of field on a wide-angle limit side is suppressed without thenecessity of a large curvature, in particular, a large curvature of theimage side surface, so that the edge thickness is easily ensured. Thus,the thickness of the first lens unit can be reduced. This reduces thethickness of the entire zoom lens system and hence the overall opticallength at the time of non-use.

The conditions (III-4) and (III-5) set forth the refractive index andthe Abbe number of the object side lens element constituting the firstlens unit. The condition (III-6) is necessary for satisfactorilycompensating chromatic aberration of a zoom lens system in which thefirst lens unit is a negative lead type and has negative optical power,and in which the first lens unit is composed of an object side lenselement having negative optical power and an image side lens elementhaving positive optical power. When these conditions (III-4), (III-5)and (III-6) are satisfied, a possibility is avoided that the opticalaxial thickness of the lens element increases with increasing light rayheight and that when the center thickness is increased for the purposeof improvement in manufacturability, the thickness of the entire firstlens unit increases further. At the same time, chromatic aberration canbe compensated satisfactorily.

Here, when at least one of the following conditions (III-2)′, (III-3)′,(III-4)′, (III-5)′ and (III-6)′ is further satisfied, theabove-mentioned effect is achieved more successfully. In particular,when the condition (III-2)′ described below is satisfied, the image sidelens element of the first lens unit can have a high Z-value (differencebetween curvature of the object side surface and curvature of the imageside surface) so that the centering of the lens becomes easier. Further,in particular, when the condition (III-3)′ described below is satisfied,chromatic aberration generated in the first lens unit can be compensatedmore satisfactorily. Further, when the following condition (III-5)″ issatisfied, chromatic aberration generated in the first lens unit can becompensated yet more satisfactorily.

n12>1.95  (III-2)′

ν12<23  (III-3)′

n11>1.70  (III-4)′

ν11>38  (III-5)′

42>ν11  (III-5)″

n12−n11>0.18  (III-6)′

Further, in a zoom lens system like the zoom lens system according toEmbodiments III-1 to III-4, it is preferable that the followingcondition (III-7) is satisfied.

T1/d12>2.0  (III-7)

where,

T1 is a center thickness of the first lens unit, and

d12 is a center interval between the object side lens element and theimage side lens element of the first lens unit.

The condition (III-7) sets forth the center thickness of the first lensunit. When the condition (III-7) is satisfied, curvature of field iscompensated satisfactorily, and a situation is avoided that the Z-valueof the object side lens element of the first lens unit increasesexcessively. Thus, in a state that a high Z-value of the image side lenselement is ensured, a possibility is avoided that the thickness of thefirst lens unit increases excessively and so does the overall opticallength at the time of non-use.

Here, when the following condition (III-7)′ is further satisfied, theabove-mentioned effect is achieved more successfully. Further, when thefollowing condition (III-7)″ is satisfied, a possibility is avoided thatthe thickness of the first lens unit increases excessively. Thus, thesize at the time of non-use can be reduced.

T1/d12>2.3  (III-7)′

3.3>T1/d12  (III-7)″

Further, in a zoom lens system like the zoom lens system according toEmbodiments III-1 to III-4, it is preferable chat the followingcondition (III-8) is satisfied.

T2/Y>0.8  (III-8)

where,

T2 is a center thickness of the second lens unit, and

Y is a maximum image height.

The condition (III-8) sets forth the center thickness of the second lensunit. When the condition (III-8) is satisfied, a situation is avoidedthat the thickness of the second lens unit becomes excessively small andthat satisfactory aberration compensation cannot be achieved.

Here, when the following condition (III-8)′ is further satisfied, theabove-mentioned effect is achieved more successfully. Further, when thefollowing condition (III-8)″ is satisfied, a possibility is avoided thatthe thickness of the second lens unit increases excessively and so doesthe overall optical length at the time of non-use.

T2/Y>0.82  (III-8)′

1.25>T2/Y  (III-8)″

Here, the lens units constituting the zoom lens system of EmbodimentsIII-1 to III-4 are composed exclusively of refractive type lens elementsthat deflect the incident light by refraction (that is, lens elements ofa type in which deflection is achieved at the interface between mediaeach having a distinct refractive index). However, the present inventionis not limited to the zoom lens system of this construction. Forexample, the lens units may employ diffractive type lens elements thatdeflect the incident light by diffraction; refractive-diffractive hybridtype lens elements that deflect the incident light by a combination ofdiffraction and refraction; or gradient index type lens elements thatdeflect the incident light by distribution of refractive index in themedium.

Further, in the zoom lens system according to Embodiments III-1 toIII-4, a reflecting surface may be arranged in the optical path so thatthe optical path may be bent before or after the zoom lens system oralternatively in the middle. The bending position may be set uparbitrarily depending on the necessity. When the optical path is bentappropriately, thickness reduction in appearance can be achieved in acamera.

Further, the zoom lens system according to Embodiments III-1 to III-4has been described for the construction that a plate P equivalent to anoptical low-pass filter is arranged between the most image side surfaceof the third lens element G3 and the image surface S. This low-passfilter may be a birefringent type low-pass filter made of, for example,a crystal whose predetermined crystal orientation is adjusted; or aphase type low-pass filter that achieves required characteristics ofoptical cut-off frequency by diffraction. Further, this plate P may bearranged depending on the necessity.

As described above, according to the present Embodiments III-1 to III-4,a zoom lens system can be obtained that has a zoom ratio of 3 orgreater, satisfactory curvature of field compensation, a small firstlens unit thickness and a reduced overall optical length at the time ofnon-use.

Embodiment 5

FIG. 25 is a schematic perspective view of a digital still cameraserving as an example of a camera according to Embodiment 5. In FIG. 25,the digital still camera comprises: a main body 901; an imaging device902 provided with a zoom lens system and an image sensor such as a CCDand a CMOS; a separate optical finder 903; a stroboscope 904; and arelease button 905. The zoom lens system in the imaging device 902 is azoom lens system according to Embodiment I-1.

As such, when the zoom lens system according to Embodiment I-1 isemployed in a digital still camera, a digital still camera can beprovided that has a reduced thickness and excellent portability as wellas high performance. Here, the digital still camera shown in FIG. 25 mayemploy any one of the zoom lens systems according to Embodiments I-2 toI-4, II-1 to II-4 and III-1 to III-4 in place of the zoom lens systemaccording to Embodiment I-1. Further, the optical system of the digitalstill camera shown in FIG. 25 is applicable also to a digital videocamera for moving images. In this case, moving images with a highresolution can be acquired in addition to still images.

Further, an imaging device comprising a zoom lens system according toEmbodiments I-1 to I-4, II-1 to II-4 and III-1 to III-4 described aboveand an image sensor such as a CCD and a CMOS may be applied to a mobiletelephone, a PDA (Personal Digital Assistance), a surveillance camera ina surveillance system, a Web camera, a vehicle-mounted camera or thelike.

Numerical examples are described below in which the zoom lens systemsaccording to Embodiments I-1 to I-4, II-1 to II-4 and III-1 to III-4 areimplemented. In the numerical examples, the units of the length in thetables are all mm. Moreover, r is the radius of curvature, d is theaxial distance, nd is the refractive index to the d-line, and νd is theAbbe number to the d-line. Further, in each numerical example, thesurface marked with * indicates an aspheric surface. The sag z of theaspheric surface is expressed by the following Formula A in Examples I-1to I-4, and by the following Formula B in Examples II-1 to II-4 andIII-1 to III-4.

$\begin{matrix}{Z = {\frac{\left( {1/{CR}} \right) \cdot H^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right) \cdot \left( {1/{CR}} \right)^{2} \cdot H^{2}}}} + {\overset{16}{\sum\limits_{n = 4}}{{An} \cdot H^{n}}}}} & {{Formula}\mspace{14mu} A}\end{matrix}$

Here, a cylindrical coordinate system is adopted in which the axis inthe direction going to the image surface side on the optical axis isdefined as the z-axis and in which the axis in a direction departingperpendicularly from the optical axis is defined as the H-axis. Further,CR is the paraxial radius of curvature (mm), K is the conic coefficient,and An is the n-th aspheric coefficient.

$\begin{matrix}{z = {\frac{{ch}^{2}}{1 + \sqrt{\left\{ {1\left( {1 + k} \right)c^{2}h^{2}} \right\}}} + {A\; h^{4}} + {Bh}^{6} + {Ch}^{8} + {Dh}^{10} + {Eh}^{12} + {Fh}^{14}}} & {{Formula}\mspace{14mu} B}\end{matrix}$

Here, h is the height from the optical axis, c is the curvature, k isthe conic constant, and A, B, C, D, E and F are the fourth-order,sixth-order, eighth-order, tenth-order, twelfth-order andfourteenth-order aspherical coefficients, respectively.

Example I-1

The zoom lens system of Example I-1 corresponds to Embodiment I-1 shownin FIG. 1. Table I-1 shows the lens data of the zoom lens system ofExample I-1. Table I-2 shows the focal length f, the F-number FNo, theoverall optical length L, the half view angle ω, and the variable axialdistance data d4, d10 and d12 in a case that the shooting distance isinfinity. Table I-3, shows the aspherical data.

TABLE I-1 Lens Lens Surface unit element number r d nd νd G1 L1 1314.449 1.100 1.80470 41.0  2* 5.283 1.671 L2  3* 9.276 1.972 1.9972020.6 4 22.774 d4 (Variable) Diaphragm 5 ∞ 0.000 G2 L3  6* 4.440 2.2721.80470 41.0 L4 7 −7.938 0.400 1.75520 27.5 8 3.865 0.721 L5  9* 13.2591.012 1.52996 55.8 10  −20.207 d10 (Variable) G3 L6 11* 18.741 1.6931.66547 55.2 12* −29.692 d12 (Variable)

TABLE I-2 Wide-angle Middle Telephoto limit position limit f (mm) 4.768.71 16.76 FNo 3.33 4.22 5.86 d4 15.78 6.63 0.60 d10 2.78 8.38 17.88 d124.34 3.92 2.60 L 25.22 23.15 25.30 ω 33.04 20.75 12.45

TABLE I-3 Surface number K A4 A6 A8 A10 A12 2 −7.20987E−01 −3.88747E−047.58009E−06 −8.06162E−07 3.16173E−08 −3.99973E−10 3 −5.66975E−01−5.56062E−05 2.02631E−06 −4.35377E−07 2.01018E−08 −2.67211E−10 6−5.63503E−01 1.75494E−04 4.60266E−05 −3.29099E−06 −5.57532E−071.26177E−07 9 0.00000E+00 −5.28657E−04 3.31108E−05 −1.13103E−044.23084E−05 −5.54498E−06 11 0.00000E+00 3.18445E−04 −1.71912E−05−1.86734E−06 1.15958E−07 −2.13155E−09 12 0.00000E+00 8.63194E−04−6.45017E−05 1.03865E−06

Example I-2

The zoom lens system of Example I-2 corresponds to Embodiment I-2 shownin FIG. 3. Table I-4 shows the lens data of the zoom lens system ofExample I-2. Table I-5 shows the focal length f, the F-number FNo, theoverall optical length L, the half view angle ω, and the variable axialdistance data d4, d10 and d12 in a case that the shooting distance isinfinity. Table I-6 shows the aspherical data.

TABLE I-4 Lens Lens Surface unit element number r d nd νd G1 L1 1290.544 1.100 1.80470 41.0  2* 5.436 1.696 L2  3* 9.716 1.972 1.9972020.6 4 23.479 d4 (Variable) Diaphragm 5 ∞ 0.000 G2 L3  6* 4.453 2.2511.80470 41.0 L4 7 −9.203 0.400 1.75520 27.5 8 3.869 0.710 L5  9* 14.4281.029 1.52996 55.8 10  −16.526 d10 (Variable) G3 L6 11* 19.838 1.6081.66547 55.2 12* −29.155 d12 (Variable)

TABLE I-5 Wide-angle Middle Telephoto limit position limit f (mm) 4.758.71 15.92 FNo 3.33 4.22 5.86 d4 15.79 6.73 1.94 d10 2.70 8.55 17.88 d124.48 3.89 2.57 L 25.04 21.96 25.36 ω 33.98 20.84 12.90

TABLE I-6 Surface number K A4 A6 A8 A10 A12 2 −6.81743E−01 −3.15467E−046.63469E−06 −7.73202E−07 2.98635E−08 −3.65093E−10 3 −4.44787E−01−3.41487E−05 3.22253E−06 −4.91718E−07 2.06445E−08 −2.57922E−10 6−5.62730E−01 1.84798E−04 2.79632E−05 −2.43430E−06 −3.79770E−071.26097E−07 9 0.00000E+00 −5.22256E−04 1.60055E−04 −1.21439E−043.54027E−05 −4.08827E−06 11 0.00000E+00 3.49196E−04 −1.59870E−05−1.77726E−06 1.13158E−07 −2.25382E−09 12 0.00000E+00 9.14216E−04−5.94809E−05 8.65461E−07

Example I-3

The zoom lens system of Example I-3 corresponds to Embodiment I-3 shownin FIG. 5. Table I-7 shows the lens data of the zoom lens system ofExample I-3. Table I-8 shows the focal length f, the F-number FNo, theoverall optical length L, the half view angle ω, and the variable axialdistance data d4, d10 and d12 in a case that the shooting distance isinfinity. Table I-9 shows the aspherical data.

TABLE I-7 Lens Lens Surface unit element number r d nd νd G1 L1 1 70.9651.000 1.80470 41.0  2* 5.427 1.810 L2 3 9.148 1.860 1.99720 20.6  4*17.107 d4 (Variable) Diaphragm 5 ∞ 0.000 G2 L3  6* 4.114 2.125 1.8047041.0 L4 7 −387.635 0.400 1.78472 25.7 8 3.500 0.430 L5  9* 11.497 1.4001.52996 55.8 10  −13.861 d10 (Variable) G3 L6 11  22.410 1.838 1.6654755.2 12* −23.299 d12 (Variable)

TABLE I-8 Wide-angle Middle Telephoto limit position limit f (mm) 4.898.94 16.82 FNo 3.33 4.22 5.86 d4 14.99 6.53 1.79 d10 2.99 8.79 17.97 d124.14 3.56 2.48 L 24.66 21.89 25.35 ω 35.00 21.00 13.35

TABLE I-9 Surface number K A4 A6 A8 A10 A12 2 −5.06216E−01 −7.77575E−05−3.72675E−06 −9.35176E−07 4.83246E−08 −7.27227E−10 4 1.97810E−01−9.96361E−05 1.24089E−05 −1.05549E−07 −1.50045E−08 3.50971E−10 6−3.34609E−01 −5.57901E−05 −9.92799E−05 6.44294E−05 −1.50265E−051.22579E−06 9 0.00000E+00 −8.27843E−05 −8.14616E−06 0.00000E+000.00000E+00 0.00000E+00 12 0.00000E+00 1.95523E−04 −5.80844E−06−1.06764E−06 1.15290E−07 −3.18056E−09

Example I-4

The zoom lens system of Example I-4 corresponds to Embodiment I-4 shownin FIG. 7. Table I-10 shows the lens data of the zoom lens system ofExample I-4. Table I-11 shows the focal length f, the F-number FNo, theoverall optical length L, the half view angle ω, and the variable axialdistance data d4, d10 and d12 in a case that the shooting distance isinfinity. Table I-12 shows the aspherical data.

TABLE I-10 Lens Lens Surface unit element number r d nd νd G1 L1 1300.000 1.100 1.80470 41.0  2* 5.281 1.674 L2  3* 9.265 1.965 1.9972020.6 4 22.687 d4 (Variable) Diaphragm 5 ∞ 0.000 G2 L3  6* 4.438 2.2761.80470 41.0 L4 7 −7.816 0.400 1.75520 27.5 8 3.866 0.721 L5  9* 13.6111.007 1.52996 55.8 10  −19.473 d10 (Variable) G3 L6 11* 18.777 1.6371.66547 55.2 12* −29.474 d12 (Variable)

TABLE I-11 Wide-angle Middle Telephoto limit position limit f (mm) 4.768.70 15.91 FNo 3.33 4.22 5.86 d4 15.72 6.65 1.93 d10 2.78 8.44 17.87 d124.34 3.87 2.55 L 25.41 20.52 25.37 ω 32.35 21.04 12.25

TABLE I-12 Surface number K A4 A6 A8 A10 A12 2 −7.13953E−01 −3.66962E−046.98415E−06 −8.06128E−07 3.17119E−08 −4.09395E−10 3 −5.59363E−01−5.61116E−05 2.42043E−06 −4.40355E−07 1.98271E−08 −2.68197E−10 6−5.58819E−01 1.84074E−04 4.56058E−05 −3.48862E−06 −5.73009E−071.31573E−07 9 0.00000E+00 −5.66137E−04 2.59802E−05 −1.11568E−044.27182E−05 −5.68923E−06 11 0.00000E+00 3.24793E−04 −1.82952E−05−1.74384E−06 1.19184E−07 −2.44258E−09 12 0.00000E+00 8.14331E−04−6.08137E−05 9.65844E−07

The following Table I-13 shows values corresponding to the conditions inExamples I-1 to I-4.

TABLE I-13 Example Condition I-1 I-2 I-3 I-4 (I-1) |(R_(2mi1) +R_(2mi2))/ 0.21 0.07 0.09 0.18 (R_(2mi1) − R_(2mi2))| (I-2) (T_(G1) +T_(G2) + 2.28 2.27 2.23 2.27 T_(G3))/f_(w) (I-3) T₁/f_(w) 0.35 0.36 0.370.35 (I-4) (R₁₂ + R₂₁)/ −3.65 −3.54 −3.92 −3.65 (R₁₂ − R₂₁) (I-5) nd₂1.99720 1.99720 1.99720 1.99720 (I-6) φ_(A)/φ_(C) 3.12 2.95 2.37 3.13(I-7) φ_(Aa)/φ_(Ca) 0.22 0.20 0.24 0.21

Example II-1

The zoom lens system of Example II-1 corresponds to Embodiment II-1shown in FIG. 9. Table II-1 shows the lens data of the zoom lens systemof Example II-1. Table II-2 shows the focal length f, the F-number, theview angle 2ω, the overall optical length L, and the variable axialdistance data d4, d10 and d12 in a case that the shooting distance isinfinity. Table II-3 shows the aspherical data.

TABLE II-1 Lens Lens unit element Surface r d nd νd G1 L1  1 45.4320.900 1.80470 41.0  2* 4.525 1.565 L2  3 8.154 1.523 1.99720 20.6  4*16.653 d4 Diaphragm  5 ∞ 0.670 G2 L3  6* 3.240 1.374 1.80470 41.0 L4  7−40.880 0.400 1.76182 26.6  8 2.789 0.530 L5  9* 12.905 1.080 1.5299655.8 10 −9.762 d10 G3 L6 11 21.168 1.000 1.66547 55.2  12* −30.731 d12 P13 ∞ 0.500 1.51680 64.2 14 0.500 P 15 ∞ 0.370 1.51680 64.2 16

TABLE II-2 Wide-angle Middle Telephoto limit position limit f 4.68 10.2414.85 F-number 2.81 4.30 5.41 2ω 77.44 38.72 26.98 L 28.36 25.92 27.32d4 11.43 3.20 0.60 d10 3.22 10.80 15.23 d12 3.56 1.76 1.32

TABLE II-3 Surface K A B C D E 2 −6.13686E−01 2.55205E−05 1.56227E−05−1.45690E−06 3.50784E−08 4 0.00000E+00 −1.47520E−04 2.76760E−061.65438E−07 −1.11449E−08 6 −3.26065E−01 −1.72587E−04 −4.59345E−051.77750E−05 −1.55466E−06 9 0.00000E+00 −4.59216E−05 5.78142E−05−1.29258E−05 0.00000E+00 12 0.00000E+00 4.52432E−04 −5.09522E−053.82711E−06 −1.43312E−07 2.01092E−09

Example II-2

The zoom lens system of Example II-2 corresponds to Embodiment II-2shown in FIG. 11. Table II-4 shows the lens data of the zoom lens systemof Example II-2. Table II-5 shows the focal length f, the F-number, theview angle 2ω, the overall optical length L, and the variable axialdistance data d4, d12 and d14 in a case that the shooting distance isinfinity. Table II-6 shows the aspherical data.

TABLE II-4 Lens Lens unit element Surface r d nd νd G1 L1 1 120.0001.060 1.80470 41.0  2* 5.422 2.158 L2  3* 13.035 1.764 1.99720 20.6 441.661 d4 Diaphragm 5 ∞ 0.670 G2 L3  6* 4.827 1.389 1.80470 41.0 726.757 0.305 L4 8 27.097 0.916 1.72916 54.7 L5 9 −15.495 0.400 1.7552027.5 10  4.220 0.497 L6 11* 15.897 0.892 1.52996 55.8 12  −11.144 d12 G3L7 13  23.104 1.520 1.66547 55.2 14* −30.387 d14 P ∞ 0.500 1.51680 64.20.500 P ∞ 0.370 1.51680 64.2

TABLE II-5 Wide-angle Middle Telephoto limit position limit f 4.77 8.7216.22 F-number 2.93 3.98 5.87 2ω 76.47 45.42 24.95 L 35.18 32.10 34.84d4 14.45 5.98 0.96 d12 2.64 9.26 18.74 d14 4.75 3.50 1.76

TABLE II-6 Surface K A B 2 −3.95446E+00 2.79257E−03 −1.30036E−04 3−1.93004E+01 1.31757E−03 −6.73598E−05 6 −7.20684E−01 2.30291E−041.02773E−04 11  0.00000E+00 −2.64377E−04 −4.45967E−05 14  0.00000E+003.24970E−04 −6.33529E−05 Surface C D E 2 5.53364E−06 −1.39941E−071.45996E−09 3 3.02681E−06 −7.74520E−08 8.09353E−10 6 −3.91601E−058.41609E−06 −6.70427E−07 11  4.81903E−08 9.20816E−08 14  7.12831E−06−3.80055E−07 7.60719E−09

Example II-3

The zoom lens system of Example II-3 corresponds to Embodiment II-3shown in FIG. 13. Table II-7 shows the lens data of the zoom lens systemof Example II-3. Table II-8 shows the focal length f, the F-number, theview angle 2ω, the overall optical length L, and the variable axialdistance data d4, d10 and d12 in a case that the shooting distance isinfinity. Table II-9 shows the aspherical data.

TABLE II-7 Lens Lens unit element Surface r d nd νd G1 L1  1 307.6611.100 1.80470 41.0  2* 5.254 1.683 L2  3* 9.219 1.955 1.99720 20.6  422.591 d4 Diaphragm  5 ∞ 0.670 G2 L3  6* 4.480 2.301 1.80470 41.0 L4  7−7.549 0.400 1.75520 27.5  8 3.920 0.710 L5  9* 14.147 0.995 1.5299655.8 10 −19.106 d10 G3 L6 11* 19.511 1.638 1.66547 55.2 12* −28.417 d12P 13 ∞ 0.500 1.51680 64.2 14 0.500 P 15 ∞ 0.370 1.51680 64.2 16

TABLE II-8 Wide-angle Middle Telephoto limit position limit f 4.76 8.7115.84 F-number 2.93 3.91 5.66 2ω 76.86 45.12 25.23 L 35.46 31.80 35.01d4 15.09 6.14 1.26 d10 2.74 8.62 17.77 d12 4.41 3.82 2.75

TABLE II-9 Surface K A B 2 −7.10368E−01 −3.52400E−04 6.40741E−06 3−5.38947E−01 −5.21717E−05 2.56968E−06 6 −5.60195E−01 1.82543E−044.39672E−05 9 0.00000E+00 −6.03956E−04 1.23408E−05 11  0.00000E+003.35621E−04 −1.83460E−05 12  0.00000E+00 7.88164E−04 −5.73977E−05Surface C D E 2 −8.03851E−07 3.19052E−08 −4.21229E−10 3 −4.49011E−071.95470E−08 −2.63024E−10 6 −3.82639E−06 −5.67027E−07 1.48819E−07 9−1.08136E−04 4.36520E−05 −6.05759E−06 11  −1.68321E−06 1.20126E−07−2.56431E−09 12  9.14905E−07

Example II-4

The zoom lens system of Example II-4 corresponds to Embodiment II-4shown in FIG. 15. Table II-10 shows the lens data of the zoom lenssystem of Example II-4. Table II-11 shows the focal length f, theF-number, the view angle 2ω, the overall optical length L, and thevariable axial distance data d4, d12 and d14 in a case that the shootingdistance is infinity. Table II12 shows the aspherical data.

TABLE II-10 Lens Lens unit element Surface r d nd νd G1 L1  1 120.0001.060 1.80470 41.0  2* 5.311 1.990 L2  3* 11.028 1.784 1.99575 20.7  427.791 d4 Diaphragm  5 ∞ 0.350 G2 L3  6* 4.973 1.256 1.80434 40.8  715.437 0.384 L4  8 10.474 0.947 1.72916 54.7 L5  9 −28.404 0.400 1.7618226.6 10 4.094 0.496 L6 11* 21.633 0.917 1.53113 55.7 12 −10.573 d12 G3L7 13* 34.991 1.520 1.66547 55.2 14* −20.585 d14 P 15 ∞ 0.500 1.5168064.2 16 0.500 P 17 ∞ 0.370 1.51680 64.2 18

TABLE II-11 Wide-angle Middle Telephoto limit position limit f 4.77 8.7115.92 F-number 2.92 3.94 5.74 2ω 76.36 45.45 25.44 L 34.89 31.63 34.48d4 14.60 6.17 1.36 d12 2.79 8.90 17.96 d14 4.74 3.81 2.42

TABLE II-12 Sur- face K A B C  2 −1.50000E+00 9.45283E−04 −4.13637E−051.60905E−06  3 0.00000E+00 2.34229E−04 −2.45453E−05 1.07278E−06  60.00000E+00 −6.50053E−04 6.34626E−05 −3.72453E−05 11 0.00000E+00−6.34185E−05 1.88100E−04 −6.38471E−05 13 0.00000E+00 1.28204E−04−3.01444E−05 1.04004E−06 14 0.00000E+00 4.95301E−04 −9.37727E−057.62691E−06 Surface D E F  2 −1.04194E−08 −8.41442E−10 1.47350E−11  3−1.18237E−08 −3.61655E−10 7.45807E−12  6 1.05395E−05 −1.61108E−061.01352E−07 11 8.37486E−06 13 14 −3.31231E−07 6.39178E−09

The following Table II-13 shows values corresponding to the conditionsin Examples II-1 to II-4.

TABLE II-13 Example Condition II-1 II-2 II-3 II-4 (II-1) f₂/f_(2r) 0.851.02 0.73 0.92 (II-2) f_(w)/f_(2r) 0.44 0.38 0.31 0.35 (II-3) T2/Y 0.940.84 1.23 0.83 (II-4) Rc/Y −11.36 −4.30 −2.10 −7.89 (II-5) (T1 + T2 +T3)/Y 2.33 2.47 3.00 2.43 (II-6) f_(3r)/f_(T) 1.28 1.23 1.11 1.24 (II-7)T3/Y 0.28 0.25 0.46 0.25

Example III-1

The zoom lens system of Example III-1 corresponds to Embodiment III-1shown in FIG. 17. Table III-1 shows the lens data of the zoom lenssystem of Example III-1. Table III-2 shows the focal length f, theF-number, the view angle 2ω, the overall optical length L, and thevariable axial distance data d4, d10 and d12 in a case that the shootingdistance is infinity. Table III-3 shows the aspherical data.

TABLE III-1 Lens Lens unit element Surface r d nd νd G1 L1  1 106.7701.007 1.80470 41.0  2* 5.186 1.177 L2  3 7.951 1.600 1.99720 20.6  4*14.443 d4 Diaphragm  5 ∞ 0.300 G2 L3  6* 2.847 1.000 1.80470 41.0 L4  75.065 0.400 1.84666 23.8  8 2.542 0.680 L5  9 9.657 1.050 1.49700 81.610 −11.474 d10 G3 L6 11 −58.781 1.100 1.66547 55.2 12* −10.516 d12 P 13∞ 0.500 1.51680 64.2 14 0.500 P 15 ∞ 0.370 1.51680 64.2 16

TABLE III-2 Wide-angle Middle Telephoto limit position limit f 4.6510.30 16.24 F-number 2.91 4.52 6.09 2ω 77.82 38.19 24.73 L 31.54 30.0532.46 d4 13.86 5.39 2.47 d10 2.71 11.81 18.54 d12 4.88 2.75 1.34

TABLE III-3 Surface K A B 2 −7.26222E−01 4.01737E−04 −1.05320E−05 40.00000E+00 −2.82235E−04 1.51056E−05 6 −6.18744E−01 1.66953E−032.78768E−04 12  0.00000E+00 5.28697E−04 −1.14171E−05 Surface C D E 24.99778E−07 −1.53649E−08 4 −6.74277E−07 1.67210E−08 6 −5.69567E−051.68321E−05 −1.27163E−06 12  2.99725E−07 5.02100E−08 −2.91521E−09

Example III-2

The zoom lens system of Example III-2 corresponds to Embodiment III-2shown in FIG. 19. Table III-4 shows the lens data of the zoom lenssystem of Example III-2. Table III-5 shows the focal length f, theF-number, the view angle 2ω, the overall optical length L, and thevariable axial distance data d4, d12 and d14 in a case that the shootingdistance is infinity. Table III-6 shows the aspherical data.

TABLE III-4 Lens Lens unit element Surface r d nd νd G1 L1  1 120.0001.060 1.80470 41.0  2* 5.422 2.158 L2  3* 13.035 1.764 1.99720 20.6  441.661 d4 Diaphragm  5 ∞ 0.670 G2 L3  6* 4.827 1.389 1.80470 41.0  726.757 0.305 L4  8 27.097 0.916 1.72916 54.7 L5  9 −15.495 0.400 1.7618226.6 10 4.257 0.497 L6 11* 18.178 0.892 1.60602 57.4 12 −12.743 d12 G3L7 13* 23.104 1.520 1.66547 55.2 14 −30.387 d14 P 15 ∞ 0.500 1.5168064.2 16 0.500 P 17 ∞ 0.370 1.51680 64.2 18

TABLE III-5 Wide-angle Middle Telephoto limit position limit f 4.76 8.6515.71 F-number 2.92 3.95 5.69 2ω 76.57 45.73 25.74 L 35.18 32.10 34.84d4 14.45 5.98 0.96 d12 2.51 8.98 17.94 d14 4.88 3.78 2.56

TABLE III-6 Surface K A B 2 −3.95446E+00 2.79257E−03 −1.30036E−04 3−1.93004E+01 1.31757E−03 −6.73598E−05 6 −7.20684E−01 2.30291E−041.02773E−04 11  0.00000E+00 −2.64377E−04 −4.45967E−05 13  0.00000E+003.24970E−04 −6.33529E−05 Surface C D E 2 5.53364E−06 −1.39941E−071.45996E−09 3 3.02681E−06 −7.74520E−08 8.09353E−10 6 −3.91601E−058.41609E−06 −6.70427E−07 11  4.81903E−08 9.20816E−08 13  7.12831E−06−3.80055E−07 7.60719E−09

Example III-3

The zoom lens system of Example III-3 corresponds to Embodiment III-3shown in FIG. 21. Table III-7 shows the lens data of the zoom lenssystem of Example III-3. Table III-8 shows the focal length f, theF-number, the view angle 2ω, the overall optical length L, and thevariable axial distance data d4, d10 and d12 in a case that the shootingdistance is infinity. Table III-9 shows the aspherical data.

TABLE III-7 Lens Lens unit element Surface r d nd νd G1 L1  1 307.6611.100 1.80470 41.0  2* 5.254 1.683 L2  3* 9.219 1.955 1.99720 20.6  422.591 d4 Diaphragm  5 ∞ 0.670 G2 L3  6* 4.480 2.301 1.80470 41.0 L4  7−7.549 0.400 1.75520 27.5  8 3.920 0.710 L5  9* 14.147 0.995 1.5299655.8 10 −19.106 d10 G3 L6 11* 19.511 1.638 1.66547 55.2 12* −28.417 d12P 13 ∞ 0.500 1.51680 64.2 14 0.500 P 15 ∞ 0.370 1.51680 64.2 16

TABLE III-8 Wide-angle Middle Telephoto limit position limit f 4.76 8.7115.84 F-number 2.93 3.91 5.66 2ω 76.86 45.12 25.23 L 35.46 31.80 35.01d4 15.09 6.14 1.26 d10 2.74 8.62 17.77 d12 4.41 3.82 2.75

TABLE III-9 Surface K A B 2 −7.10368E−01 −3.52400E−04 6.40741E−06 3−5.38947E−01 −5.21717E−05 2.56968E−06 6 −5.60195E−01 1.82543E−044.39672E−05 9 0.00000E+00 −6.03956E−04 1.23408E−05 11  0.00000E+003.35621E−04 −1.83460E−05 12  0.00000E+00 7.88164E−04 −5.73977E−05Surface C D E 2 −8.03851E−07 3.19052E−08 −4.21229E−10 3 −4.49011E−071.95470E−08 −2.63024E−10 6 −3.82639E−06 −5.67027E−07 1.48819E−07 9−1.08136E−04 4.36520E−05 −6.05759E−06 11  −1.68321E−06 1.20126E−07−2.56431E−09 12  9.14905E−07

Example III-4

The zoom lens system of Example III-4 corresponds to Embodiment III-4shown in FIG. 23. Table III-10 shows the lens data of the zoom lenssystem of Example III-4. Table III-11 shows the focal length f, theF-number, the view angle 2ω, the overall optical length L, and thevariable axial distance data d4, d12 and d14 in a case that the shootingdistance is infinity. Table III-12 shows the aspherical data.

TABLE III-10 Lens Lens unit element Surface r d nd νd G1 L1  1 120.0001.060 1.80470 41.0  2* 5.311 1.990 L2  3* 11.028 1.784 1.99575 20.7  427.791 d4 Diaphragm  5 ∞ 0.350 G2 L3  6* 4.973 1.256 1.80434 40.8  715.437 0.384 L4  8 10.474 0.947 1.72916 54.7 L5  9 −28.404 0.400 1.7618226.6 10 4.094 0.496 L6 11* 21.633 0.917 1.53113 55.7 12 −10.573 d12 G3L7 13* 34.991 1.520 1.66547 55.2 14* −20.585 d14 P 15 ∞ 0.500 1.5168064.2 16 0.500 P 17 ∞ 0.370 1.51680 64.2 18

TABLE III-11 Wide-angle Middle Telephoto limit position limit f 4.778.71 15.92 F-number 2.92 3.94 5.74 2ω 76.36 45.45 25.44 L 34.89 31.6334.48 d4 14.60 6.17 1.36 d12 2.79 8.90 17.96 d14 4.74 3.81 2.42

TABLE III-12 Sur- face K A B C  2 −1.50000E+00 9.45283E−04 −4.13637E−051.60905E−06  3 0.00000E+00 2.34229E−04 −2.45453E−05 1.07278E−06  60.00000E+00 −6.50053E−04 6.34626E−05 −3.72453E−05 11 0.00000E+00−6.34185E−05 1.88100E−04 −6.38471E−05 13 0.00000E+00 1.28204E−04−3.01444E−05 1.04004E−06 14 0.00000E+00 4.95301E−04 −9.37727E−057.62691E−06 Surface D E F  2 −1.04194E−08 −8.41442E−10 1.47350E−11  3−1.18237E−08 −3.61655E−10 7.45807E−12  6 1.05395E−05 −1.61108E−061.01352E−07 11 8.37486E−06 13 14 −3.31231E−07 6.39178E−09

The following Table III-13 shows values corresponding to the conditionsin Examples III-1 to III-4.

TABLE III-13 Example Condition III-1 III-2 III-3 III-4 (III-1) f₂/f_(w)2.15 2.69 2.37 2.60 (III-2) n12 1.99720 1.99720 1.99720 1.99575 (III-3)ν12 20.60 20.60 20.60 20.70 (III-4) n11 1.80470 1.80470 1.80470 1.80470(III-5) ν11 41.00 41.00 41.00 41.00 (III-6) n12 − n11 0.19 0.19 0.190.19 (III-7) T1/d12 3.21 2.31 2.82 2.43 (III-8) T2/Y 0.87 0.84 1.23 0.83

FIG. 2 is a longitudinal aberration diagram of the zoom lens systemaccording to Example I-1. FIG. 4 is a longitudinal aberration diagram ofthe zoom lens system according to Example I-2. FIG. 6 is a longitudinalaberration diagram of the zoom lens system according to Example I-3.FIG. 8 is a longitudinal aberration diagram of the zoom lens systemaccording to Example I-4.

FIG. 10 is a longitudinal aberration diagram of the zoom lens systemaccording to Example II-1. FIG. 12 is a longitudinal aberration diagramof the zoom lens system according to Example II-2. FIG. 14 is alongitudinal aberration diagram of the zoom lens system according toExample II-3. FIG. 16 is a longitudinal aberration diagram of the zoomlens system according to Example II-4.

FIG. 18 is a longitudinal aberration diagram of the zoom lens systemaccording to Example III-1. FIG. 20 is a longitudinal aberration diagramof the zoom lens system according to Example III-2. FIG. 22 is alongitudinal aberration diagram of the zoom lens system according toExample III-3. FIG. 24 is a longitudinal aberration diagram of the zoomlens system according to Example III-4.

In each longitudinal aberration diagram, part (a) shows the aberrationat a wide-angle limit, part (b) shows the aberration at an approximatelymiddle position, and part (c) shows the aberration at a telephoto limit.In each longitudinal aberration diagram, in order starting from theleft-hand side, the spherical aberration, the astigmatism, thedistortion, the axial chromatic aberration and the magnificationchromatic aberration are shown. In each spherical aberration diagram,the vertical axis indicates the F-number, and the solid line indicatesthe characteristics to the d-line. In each astigmatism diagram, thevertical axis indicates the half view angle ω, and the solid line andthe dash line indicate the characteristics to the sagittal image plane(in each Fig., indicated as “s”) and the meridional image plane (in eachFig., indicated as “m”), respectively. In each distortion diagram, thevertical axis indicates the half view angle ω. In each axial chromaticaberration diagram, the vertical axis indicates the F-number, and, thesolid line, the short dash line and the long dash line indicate thecharacteristics to the d-line, the F-line and the C-line, respectively.In each magnification chromatic aberration diagram, the vertical axisindicates the half view angle ω, and the short dash line and the longdash line indicate the characteristics to the F-line and the C-line,respectively.

As seen from the longitudinal aberration diagrams of FIGS. 2, 4, 6 and8, the zoom lens systems of Examples I-1 to I-4 have sufficientaberration compensation capability for achieving a high resolution.

As seen from the longitudinal aberration diagrams of FIGS. 10, 12, 14and 16, the zoom lens systems of Examples II-1 to II-4 have sufficientaberration compensation capability for achieving a high resolution.

As seen from the longitudinal aberration diagrams of FIGS. 18, 20, 22and 24, the zoom lens systems of Examples III-1 to III-4 have sufficientaberration compensation capability for achieving a high resolution.

The zoom lens system according to the present invention is applicable toa camera such as a digital still camera, a digital video camera, amobile telephone, a PDA (Personal Digital Assistance), a surveillancecamera in a surveillance system, a Web camera and a vehicle-mountedcamera. In particular, the present zoom lens system is suitable for athin camera such as a digital still camera and a digital video camerarequiring high image quality.

Details of the present invention have been described above. However, theabove-mentioned description is completely illustrative from every pointof view, and does not limit the scope of the present invention.Obviously, various improvements and modifications can be performedwithout departing from the scope of the present invention.

1-11. (canceled)
 12. A zoom lens system that forms an optical image ofan object with variable magnification and that, in order from the objectside to the image side, comprises: a first lens unit having negativeoptical power; a second lens unit having positive optical power; and athird lens unit having positive optical power, wherein in zooming from awide-angle limit to a telephoto limit, the lens units move respectivelyalong an optical axis in such a manner that an interval between thefirst lens unit and the second lens unit should decrease while aninterval between the second lens unit and the third lens unit shouldvary so that the variable magnification is achieved, the first lens unitis composed of one object side lens element and one image side lenselement, the image side lens element of the first lens unit has positiveoptical power, the most image side lens element constituting the secondlens unit is a lens element composed of a resin and having at least oneaspheric surface, and the following condition (II-1) is satisfied:0.7<f ₂ /f _(2r)<1.5  (II-1) where, f₂ is a composite focal length ofthe second lens unit, and f_(2r) is a focal length of the most imageside lens element of the second lens unit.
 13. The zoom lens system asclaimed in claim 12, satisfying the following condition (II-2):0.2<f _(W) /f _(2r)<0.6  (II-2) (here, ω_(W)>37) where, f_(W) is a focallength of the entire system at a wide-angle limit, f_(2r) is a focallength of the most image side lens element of the second lens unit, andω_(W) is a half view angle at a wide-angle limit.
 14. The zoom lenssystem as claimed in claim 12, satisfying the following condition(II-3):T2/Y>0.8  (II-3) where, T2 is a center thickness of the second lensunit, and Y is a maximum image height.
 15. The zoom lens system asclaimed in claim 12, wherein the second lens unit has at least acemented lens element composed of a positive lens element and a negativelens element, and wherein the following conditions (II-4) and (II-5) aresatisfied:−20<Rc/Y<−1  (II-4)2.0<(T1+T2+T3)/Y<3.5  (II-5) where, Rc is a radius of curvature of acemented surface of the cemented lens element in the second lens unit, Yis a maximum image height, T1 is a center thickness of the first lensunit, T2 is a center thickness of the second lens unit, and T3 is acenter thickness of the third lens unit.
 16. The zoom lens system asclaimed in claim 12, wherein the third lens unit is composed of onebi-convex lens element having at least one aspheric surface, and whereinthe following conditions (II-6) and (II-7) are satisfied:f _(3r) /f _(T)<1.5  (II-6)T3/Y<0.8  (II-7) (here, ω_(T)<15) where, f_(3r) is a focal length of thebi-convex lens element of the third lens unit, f_(T) is a focal lengthof the entire system at a telephoto limit, T3 is a center thickness ofthe third lens unit, Y is a maximum image height, and ω_(T) is a halfview angle at a telephoto limit.
 17. An imaging device capable ofconverting an optical image of a photographic object into an electricimage signal and then outputting the signal, comprising: a zoom lenssystem that forms the optical image of the photographic object withvariable magnification; and an image sensor that converts the opticalimage formed by the zoom lens system into the electric image signal,wherein in the zoom lens system, the system, in order from the objectside which is the photographic object side to the image side, comprises:a first lens unit having negative optical power; a second lens unithaving positive optical power; and a third lens unit having positiveoptical power, wherein in zooming from a wide-angle limit to a telephotolimit, the lens units move respectively along an optical axis in such amanner that an interval between the first lens unit and the second lensunit should decrease while an interval between the second lens unit andthe third lens unit should vary so that the variable magnification isachieved, the first lens unit is composed of one object side lenselement and one image side lens element, the image side lens element ofthe first lens unit has positive optical power, the most image side lenselement constituting the second lens unit is a lens element composed ofa resin and having at least one aspheric surface, and the followingcondition (II-1) is satisfied:0.7<f ₂ /f _(2r)<1.5  (II-1) where, f₂ is a composite focal length ofthe second lens unit, and f_(2r) is a focal length of the most imageside lens element of the second lens unit.
 18. A camera capable ofshooting a photographic object and then outputting its image as anelectric image signal, comprising an imaging device including a zoomlens system that forms the optical image of the photographic object withvariable magnification and an image sensor that converts the opticalimage of the photographic object formed by the zoom lens system into theelectric image signal, wherein in the zoom lens system, the system, inorder from the object side which is the photographic object side to theimage side, comprises: a first lens unit having negative optical power;a second lens unit having positive optical power; and a third lens unithaving positive optical power, wherein in zooming from a wide-anglelimit to a telephoto limit, the lens units move respectively along anoptical axis in such a manner that an interval between the first lensunit and the second lens unit should decrease while an interval betweenthe second lens unit and the third lens unit should vary so that thevariable magnification is achieved, the first lens unit is composed ofone object side lens element and one image side lens element, the imageside lens element of the first lens unit has positive optical power, themost image side lens element constituting the second lens unit is a lenselement composed of a resin and having at least one aspheric surface,and the following condition (II-1) is satisfied:0.7<f ₂ /f _(2r)<1.5  (II-1) where, f₂ is a composite focal length ofthe second lens unit, and f_(2r) is a focal length of the most imageside lens element of the second lens unit. 19-26. (canceled)